Reduce build matrix from 81 jobs to 36.

The build matrix is getting way too large. Even with the high
parallelization of the matrix it still takes 2.5+ hours of real time to
complete.

With the release of Ruby 2.1.5 we see the build matrix grow by another
nine combinations, increasing from 72 to 81 jobs. While Ruby 2.1 tends
to run faster, it still is more time to wait.

Currently, there has not been a Ruby 2.1.x specific bug reported for any
of the 2.1 releases. Ruby it attempting to follow close to SemVer for
the 2.1+ releases. With this change, I feel it is relatively safe to
adopt this lighter build matrix.

If an issue does appear which is specific to a single 2.1.x release, we
can easily add it back. If it only occurs with a specific Rails version
then we can use the `include` feature to only add back as few jobs as
necessary.
Merge pull request #1223 from rspec/reduce-build-matrix

Reduce build matrix from 81 jobs to 36.

[ci skip]