Pin Travis test config to Ubuntu Precise.
Pin Travis test config to Ubuntu Precise.

Travis CI is in the process of changing all images from Precise to Trusty.

See their announcement: https://blog.travis-ci.com/2017-07-11-trusty-as-default-linux-is-coming

This would break how we install our dependencies.

Note that we're using the Container based infrastructure (sudo: false). They plan to get rid of this combination (Precise + sudo: false) by Sep 5. Before that, we should try "sudo: required" or do the planned longterm change to run the tests within Docker images.
enisoc/vitess,rnavarro/vitess,alainjobart/vitess,sougou/vitess,davygeek/vitess,mahak/vitess,sougou/vitess,mahak/vitess,mattharden/vitess,rnavarro/vitess,mahak/vitess,mattharden/vitess,davygeek/vitess,HubSpot/vitess,tirsen/vitess,mahak/vitess,rnavarro/vitess,tinyspeck/vitess,alainjobart/vitess,mattharden/vitess,enisoc/vitess,dcadevil/vitess,tinyspeck/vitess,HubSpot/vitess,rnavarro/vitess,dcadevil/vitess,mattharden/vitess,mahak/vitess,mattharden/vitess,sougou/vitess,mattharden/vitess,davygeek/vitess,tirsen/vitess,tinyspeck/vitess,alainjobart/vitess,davygeek/vitess,alainjobart/vitess,tinyspeck/vitess,sougou/vitess,enisoc/vitess,rnavarro/vitess,alainjobart/vitess,enisoc/vitess,HubSpot/vitess,vitessio/vitess,mattharden/vitess,mattharden/vitess,vitessio/vitess,HubSpot/vitess,mattharden/vitess,davygeek/vitess,mahak/vitess,dcadevil/vitess,dcadevil/vitess,rnavarro/vitess,rnavarro/vitess,dcadevil/vitess,tirsen/vitess,mattharden/vitess,dcadevil/vitess,tirsen/vitess,vitessio/vitess,tirsen/vitess,vitessio/vitess,HubSpot/vitess,vitessio/vitess,sougou/vitess,mahak/vitess,dcadevil/vitess,tinyspeck/vitess,rnavarro/vitess,HubSpot/vitess,tirsen/vitess,alainjobart/vitess,sougou/vitess,davygeek/vitess,HubSpot/vitess,HubSpot/vitess,alainjobart/vitess,sougou/vitess,enisoc/vitess,tirsen/vitess,rnavarro/vitess,sougou/vitess,tirsen/vitess,alainjobart/vitess,tinyspeck/vitess,mahak/vitess,enisoc/vitess,enisoc/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,vitessio/vitess,davygeek/vitess,tirsen/vitess