.travis.yml: Configure Travis CI for automated testing

The installation is based on the [Ubuntu 12.04 instructions][1].  Conveniently for us, [Travis uses Ubuntu 12.04][2] as its Linux environment as well.  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][11], which is
 too old for the libraries we've installed from packages.
* 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.

For an intro to Travis, see:
* http://docs.travis-ci.com/
* http://docs.travis-ci.com/user/getting-started/

[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