Switch to Trusty on Travis CI
Switch to Trusty on Travis CI

GitHub: https://github.com/travis-ci/travis-ci/issues/7743

Ruby 1.9.2 no longer installs on Travis CI's Precise build images so
let's upgrade to Trusty instead.

As Trusty should come with a newer compiler than Precise that supports
C++11 out of the box, we can drop our dependency on clang in the build
matrix.

mudge/re2,mudge/re2