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.