Make Coveralls opt-in

Due to looking for a value of 'false' whenever I run tests locally
without the `COVERAGE` env variable coverage is very slowly, still
calculated - it then aborts without doing anything because I'm not using
a CI setup.

This adds the setting in for Travis to opt in and should exclude
coverage work for anyone running tests locally.
Reduce size of Travis matrix

We excluded almost as much as we care about and adding COVERAGE created
a number of invalid combinations so this changes the matrix to include
the workable combinations of Ruby version and Gemfile.

COVERAGE is only on Ruby 1.9.3 whilst I confirm this is working
Revert to original .travis.yml and include one case
Merge pull request #2615 from fog/coverage_opt_in

Make Coveralls opt-in