Move all tf distribution imports within try/except block

[dustinvtran]

We autogenerate random variables from tensorflow.contrib.distributions, thus relying on specific TensorFlow versions. This provides a more informative message if the user does not have a supported TensorFlow version, thus making installation issues (#569 (comment), #568) easier to solve.

Remarks

• Ideally this would be caught during installation of Edward in setup.py, but due to the tensorflow/tensorflow-gpu dichotomy we cannot explicitly depend on a TensorFlow version.
• This is not testable with Travis unless we use an outdated TensorFlow version just for this test; that's too impractical to have for all integration testing.

[dustinvtran]

From manual testing, this PR works for the following versions:

>>> print(tf.__version__)
'0.10.0'
>>> import edward as ed
Traceback (most recent call last):
  File "/Users/dvt/.pythonrc", line 11, in <module>
    import edward as ed
  File "edward/__init__.py", line 5, in <module>
    from edward import criticisms
  File "edward/criticisms/__init__.py", line 5, in <module>
    from edward.criticisms.evaluate import *
  File "edward/criticisms/evaluate.py", line 9, in <module>
    from edward.models import RandomVariable
  File "edward/models/__init__.py", line 5, in <module>
    from edward.models.dirichlet_process import *
  File "edward/models/dirichlet_process.py", line 7, in <module>
    from edward.models.random_variable import RandomVariable
  File "edward/models/random_variable.py", line 11, in <module>
    raise ImportError("{0}. Your TensorFlow version is not supported.".format(e))
ImportError: cannot import name register_session_run_conversion_functions. Your TensorFlow version is not supported.

>>> print(tf.__version__)
'0.11.0'
>>> import edward as ed
Traceback (most recent call last):
  File "/Users/dvt/.pythonrc", line 11, in <module>
    import edward as ed
  File "edward/__init__.py", line 5, in <module>
    from edward import criticisms
  File "edward/criticisms/__init__.py", line 5, in <module>
    from edward.criticisms.evaluate import *
  File "edward/criticisms/evaluate.py", line 9, in <module>
    from edward.models import RandomVariable
  File "edward/models/__init__.py", line 5, in <module>
    from edward.models.dirichlet_process import *
  File "edward/models/dirichlet_process.py", line 7, in <module>
    from edward.models.random_variable import RandomVariable
  File "edward/models/random_variable.py", line 11, in <module>
    raise ImportError("{0}. Your TensorFlow version is not supported.".format(e))
ImportError: cannot import name register_session_run_conversion_functions. Your TensorFlow version is not supported.

>>> print(tf.__version__)
'0.12.0'
>>> import edward as ed
Traceback (most recent call last):
  File "/Users/dvt/.pythonrc", line 11, in <module>
    import edward as ed
  File "edward/__init__.py", line 5, in <module>
    from edward import criticisms
  File "edward/criticisms/__init__.py", line 5, in <module>
    from edward.criticisms.evaluate import *
  File "edward/criticisms/evaluate.py", line 9, in <module>
    from edward.models import RandomVariable
  File "edward/models/__init__.py", line 5, in <module>
    from edward.models.dirichlet_process import *
  File "edward/models/dirichlet_process.py", line 13, in <module>
    raise ImportError("{0}. Your TensorFlow version is not supported.".format(e))
ImportError: First parent class declared for Bernoulli must be Distribution, but saw 'RandomVariable'. Your TensorFlow version is not supported.

>>> print(tf.__version__)  # this is the nightly build, https://ci.tensorflow.org/view/Nightly/job/nightly-matrix-cpu/TF_BUILD_IS_OPT=OPT,TF_BUILD_IS_PIP=PIP,TF_BUILD_PYTHON_VERSION=PYTHON2,label=mac-slave/lastSuccessfulBuild/artifact/pip_test/whl/tensorflow-1.0.1-py2-none-any.whl
'1.0.1'
>>> import edward as ed
Traceback (most recent call last):
  File "/Users/dvt/.pythonrc", line 11, in <module>
    import edward as ed
  File "edward/__init__.py", line 6, in <module>
    from edward import inferences
  File "edward/inferences/__init__.py", line 11, in <module>
    from edward.inferences.laplace import *
  File "edward/inferences/laplace.py", line 16, in <module>
    raise ImportError("{0}. Your TensorFlow version is not supported.".format(e))
ImportError: cannot import name MultivariateNormalCholesky. Your TensorFlow version is not supported.