(maint) travis: use non-system python
(maint) travis: use non-system python

This works around a bug in pyenv (explained here
https://github.com/pyenv/pyenv/issues/789) that breaks tools like
rvm/rbenv when using the system python.

puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb