Packaging fix

Don't require a specific BioPython version

It's not necessary to pin the BioPython dependency on a specific
version, but it may result in issues with dependency resolving in
projects using the description extractor.

http://python-packaging-user-guide.readthedocs.org/en/latest/requirements/

Describe protein

Don't require sudo on Travis

Fix setup.py to always run SWIG before copying generated files

Use `python setup.py install` instead of `pip install .` on Travis CI
so that it will catch the SWIG setup error we fixed in [1].

[1] https://github.com/mutalyzer/description-extractor/commit/3cb4333bcf02776f855569907ec381acc1907abc

We had some weird issues where running `python setup.py install` did not
correctly install the SWIG generated file. Apparently some steps where
not done in the correct order.

Installing with `python setup.py develop`, or `pip install .`, or with
pip directly from PyPI would work fined, as would running the regular
`python setup.py install` twice.

A similar issue was filed for WND-CHARM [1] and on Python distutils [2].
We implement the workaround provided in [3].

[1] https://github.com/wnd-charm/wnd-charm/issues/3
[2] http://bugs.python.org/issue7562
[3] http://stackoverflow.com/questions/12491328/python-distutils-not-include-the-swig-generated-module

PyPI installable

SWIG cleanup