Don't install documentation on travis-ci

MSP-11671

Passs `--no-ri --no-rdoc` to gem.
Don't install development gems on travis-ci

MSP-11671

Development group contains only yard+redcarpet, which is used to
generated documentation, which isn't done on travis-ci, and pry, which
is used for the msfconsole command, which isn't tested on travis-ci.
Combine cucumber and cucumber:boot into one job

MSP-11671

The run times for cucumber are so much faster now that `spec --tag
content` is actually the slowest job (on 1.9.3).  The job is so slow
that the two cucumbers can be merged together and they will take the
same time, but we'll consume 2 fewer job slots (1 for 1.9.3 and 1 for
2.1), so the longer per job time is better as long as it does become the
longest job time.