.travis.yml: Configure Travis CI for automated testing

The installation instructions are based on the Ubuntu 12.04
instructions [1].  Conveniently for us, Travis uses Ubuntu 12.04 as
its Linux environment as well [2].  I made a few changes to the
suggested installation procedure:

* build-essential [3] instead of g++ (so we get Make too)
* Non-Python [4] RDKit from packages [5,6,7,8,9] instead of from
  source, although this installs RDKit v201106, which may be too old?
  I haven't checked any of the InChl stuff [10] for building from
  source, so this might be completely broken at the moment.
  Unfortunately, the version of RDKit on PyPI is 2009.Q1b1, which is
  too old for the libraries we've installed from packages [11].
* Travis doesn't support Python 2.5 [12], so I only test on 2.6 and
  2.7.

I also don't understand how RMG-Py finds RMG-database, so that might
be broken as well.

[1]: http://greengroup.github.io/RMG-Py/users/rmg/installation/linux.html
[2]: http://docs.travis-ci.com/user/installing-dependencies/#Installing-Ubuntu-packages
[3]: http://packages.ubuntu.com/precise/build-essential
[4]: http://docs.travis-ci.com/user/languages/python/#Travis-CI-Uses-Isolated-virtualenvs
[5]: http://www.rdkit.org/docs/Install.html#ubuntu-12-04-and-later
[6]: http://packages.ubuntu.com/precise/python-rdkit
[7]: http://packages.ubuntu.com/precise/librdkit-dev
[8]: http://packages.ubuntu.com/precise/librdkit1
[9]: http://packages.ubuntu.com/precise/rdkit-data
[10]: http://code.google.com/p/rdkit/wiki/BuildingWithCmake#Getting_Ready
[11]: https://pypi.python.org/pypi/rdkit
[12]: http://docs.travis-ci.com/user/languages/python/#Choosing-Python-versions-to-test-against