fix variable names for building from source distribution

[grlee77]

I received the following error when trying to build from the source 4.6.0 tarball obtained from
https://github.com/enthought/chaco/releases/download/4.6.0/chaco-4.6.0.tar.gz

Traceback (most recent call last):
  File "setup.py", line 133, in <module>
    write_version_py()
  File "setup.py", line 113, in write_version_py
    match = re.match(r'.*?\.dev(?P<dev_num>\d+)', full_v)
NameError: name 'full_v' is not defined

This PR fixes the variable names in this section of setup.py.
Not 100% certain if it should be full_version or fullversion, but I went with the latter as that is what seemed to be used in the other cases.

(Alternately, deleting chaco/_version.py also allows the build to proceed by avoiding this section of setup.py)

[corranwebster]

Thanks for the contribution! This was entirely my fault copy pasting code to get the release out and avoid a different bug in this section.

We'll probably put out a 4.6.1 release shortly to fix the issue.

[mdickinson]

@corranwebster: Did the 4.6.1 release ever make it to PyPI?