README fixes

[olgabot]

Fix the build badge for Travis-CI. For some reason shields.io wasn't working, but the build is passing: https://travis-ci.org/YeoLab/flotilla

Not sure why the License status isn't showing, because we have the BSD license on PyPI, which is where it's getting fetched from. PyPI: https://pypi.python.org/pypi/flotilla

Badge code:

https://img.shields.io/pypi/l/flotilla.svg

Badge:

• Is it mergable?
• Did it pass the tests?
• If it introduces new functionality in scripts/ is it tested?
  Check for code coverage. To run code coverage on only the file you changed,
  for example flotilla/compute/splicing.py, use this command:
  py.test --cov flotilla/compute/splicing.py --cov-report term-missing flotilla/test/compute/test_splicing.py
  which will show you which lines aren't covered by the tests.
• Do the new functions have descriptive
  numpydoc
  style docstrings?
• If it adds a new plot, is it documented in the gallery?
• Is it well formatted? Look at make pep8 and make lint output
• Is it documented in the doc/releases/?
• Was a spellchecker run on the source code and documentation after
  changes were made?