Adding tests script to travis.yml

[marjoripomarole]

• Adding tests running on travis in every commit and PR.
• Removing coverage results from test run until issue Reported code coverage differs from what PHP/Xdebug reports facebook/hhvm#1589 is resolved
• Rearranging of hhvm install in provision script to make sure 3.13 is installed in every machine.

[gsingh93]

I'd like to remove the extra sudo apt-get update in the future, but this is fine for now.

[javuto]

Nice job @mpomarole !