[travis] Use a gcc/g++-based compiler for node native compilation.
[travis] Use a gcc/g++-based compiler for node native compilation.

This change is to work around the LLVM apt site going offline (source:
http://lists.llvm.org/pipermail/llvm-dev/2016-May/100303.html) which
caused the Travis builds to all fail (source
https://github.com/travis-ci/apt-source-whitelist/issues/279). We'll use
the gcc/g++ alternative toolchain from the
[docs](https://docs.travis-ci.com/user/languages/javascript-with-nodejs#Node.js-v4-(or-io.js-v3)-compiler-requirements).

Signed-off-by: Fletcher Nichol <fnichol@nichol.ca>

Pull request: #599
Approved by: reset

georgemarshall/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,habitat-sh/habitat,nathenharvey/habitat,habitat-sh/habitat,nathenharvey/habitat,rsertelon/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat