.travis.yml: add Python 3.4.

According to https://github.com/travis-ci/travis-ci/issues/1989 Travis
will get Python versions from
https://launchpad.net/~fkrull/+archive/deadsnakes
and that PPA has python3.4 so it should be installed to them soon.

Feel free to not merge this until it has happened.
Merge pull request #590 from Mkaysi/travis

.travis.yml: add Python 3.4.