Shell escape some of the TRAVIS_ env vars

These might potentially contain characters such as spaces and
parentheses, causing the build script to have syntax errors.

Fixes travis-ci/travis-ci#1707.
Add tests for the shell escaping of some TRAVIS_ vars

The escaping was added in ff00dada23d7f623bdd50b780218078a2d29d184.
Merge pull request #179 from travis-ci/hh-fix-env-vars

Shell escape some of the TRAVIS_ env vars