Travis - force default python to 2.7
Travis - force default python to 2.7

Travis [announced](https://changelog.travis-ci.com/upcoming-python-default-version-update-96873) a switch to python 3 as default will happen on Apr 16th, 2019.

node-gyp relies on python 2.7, without node-gyp, `yarn` can fail installing a JS dependency

=> forcing default python version to 2.7 on travis

ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic