Travis CI config: remove Python 2.5, 3.4

The Travis service isn't set up for 3.4 yet. 2.5 is no longer
present on master, so duplicate this here so we can at least
get a successful build.
Make some doctests into unit tests

Exceptions internal to pylibmc are printed with the _pylibmc prefix
in Python 3. This causes a doctest value comparison failure which
can be addressed with the IGNORE_EXCEPTION_DETAIL directive, but this
isn't available in 2.6.

Until 2.6 support is dropped, use unit tests for this functionality.