Support Ruby 1.8.7 for rspec-rails.

rspec-rails will be supporting Ruby 1.8.7 for the 3.x line. Ammeter will
need to continue support 1.8.7 if it is going to be used by rspec-rails.
The travis builds cannot fail any more on MRI versions.
Speed up bundle install on Travis.

Update the travis config per travis doc structure. Move `ruby-head` to
later in the build so that the more important versions run first. Speed
up the bundle install and improve gem install failures.