Fix PRERELEASE documentation

MSP-9816

PRERELEASE doesn't exist on master, so YARD links using {PRERELEASE} had
to be replaced with `PRERELEASE`.
Update version for branch name
Fix gem version to work with Rubygems < 2.1

Rubygems < 2.1 does not support dashes in gem versions.  Rubygems >= 2.2
automatically substitutes dashes with ".pre.".  We now emulate the
2.2 behavior so we are compatible with < 2.1.

MSP-10714
Land pr #3 bug/MSP-10714/gem-version to master
Merge branch 'master' into staging/rails-4.0

Conflicts:
	lib/metasploit/erd/version.rb