travis-ci: attempt to use Ubuntu's Python
travis-ci: attempt to use Ubuntu's Python

See https://github.com/travis-ci/travis-ci/issues/2231

astraw/stdeb,pombredanne/stdeb,benthomasson/stdeb,sathieu/stdeb,LeMeteore/stdeb,astraw/stdeb,pombredanne/stdeb,LeMeteore/stdeb,benthomasson/stdeb,sathieu/stdeb,astraw/stdeb,pombredanne/stdeb,LeMeteore/stdeb,sathieu/stdeb,benthomasson/stdeb