Improve travis build.

- No need to install bundler twice.
- No need to bundle install twice.
- Skip one-by-one specs on JRuby.
- Run cucumber using our standalone bundle
  (rather than bundle exec).
- Bundler issue #2383 has been resolved,
  no need to work around it anymore.
Update caller filter.

- Include additional libs.
- Don't put it at `rspec/caller_filter`.
- Ignore the "same content as rspec-core" spec for now.

The way I had this was brittle and is causing problems in rspec-core.
In rspec-core I'm trying to update the regex and add some specs but
the fact that the file is at the same path in all 3 libs (which seemed
like a simple way to ensure only one of them loads) is causing
the _wrong_ version to be loaded in rspec-core's specs, so that it's not
picking up the changes I've made.  Putting the file at the same
path in all 3 libs was a terrible idea in retrospect :(.
Revert how we run cucumber.

It's causing a SystemStackError on 1.9.2.
I'm not sure why and don't have time to debug it now.
Merge pull request #412 from rspec/improve_travis_build

Improve travis build.
Update caller filter once more...
Correct inaccurate doc string.
Add a warning when the should syntax is used.

Signed-off-by: Sam Phippen <samphippen@googlemail.com>