More robust `rvm env` handling

MSP-11400

Includes specs with full coverage.  Integration with aruba steps to
follow.
Steps for creating and using gemsets

MSP-11400

Break up "I'm using a clean gemset" into "I create a gemset" and "I use
a gemset" so intermediary steps can be taken between gemset creation and
use.  #use_clean_gemset is rewritten to use the underlying
calls for each respective step, #create_gemset and #use_gemset.
Fix rubocop offenses

MSP-11400
Build and install project gem steps

MSP-11400

'I build gem from project's "<gemspec path>"' will do `gem build
<gemspec path>` where <gemspec_path> is relative to PWD, which is
assumed to the project's root, and not Aruba's current directory.  'I
install latest local "<name>"' looks in the PWD for the lastest gem with
given name and installs it.  It is recommended that 'I use gemset
"<name>"' is used before the installation so that it doesn't affect the
project under test and only exists during the scenario.
Move built gem to aruba's current_dir

MSP-11400

Don't leave the built gem from 'I build gem from project's "<gemspec>"'
in the root of the project as it will show up on git status and could be
accidentally committed.  Move the gem to the aruba current directory so
it is cleaned up.
Search for #latest_local_gem in current_dir

MSP-11400

Since #build_gem now moves the built gem to #current_dir,
 #latest_local_gem should search for the gem #in_current_dir.
Build gem in project gemset instead of clean gemset

MSP-11400

It shouldn't be expected that a gem can be built outside its native
gemset, so reorder steps so building the gem happens before using a
clean gemset.
Fix timeout precedence

jruby.rb and env.rb both set @aruba_timeout_seconds, so the load order,
which is OS-dependent, affected whether the jruby timeout of 15 seconds
or the @gem-install timeout of 5 minutes was being used for jruby.
Centralized all timeouts to timeout.rb so these loading issues don't
occur again.
Use pre-release childprocess for JRuby compatibility

PR is up against upstream childprocess, so use PR branch for now to gain
ability to unset environment variables in JRuby ChildProcess when
ENV['foo'] = nil.
Use correct symbol for github shorthand

MSP-11400
Fix rubocop offenses

MSP-11400
Use tag for pre-release childprocess

MSP-11400

Use tag instead of branch as tag won't be deleted, but branch will once
it's no longer needed for PR.
General @gem-install to @slow

MSP-11400

Both `gem install` and `gem build` are slow, so just make a @slow tag
instead of @gem-install.
Bump version number

MSP-11400

To allow tagging for metasploit-version.
Update childprocess to released version

MSP-11400
Merge pull request #2 from rapid7/feature/MSP-11400/metasploit-yard-port

Port Metasploit::Yard::Aruba