Load environment before starting msfgui

msfgui checks for the msgpack gem before spinning up, however if this
gem is installed in lib/gemcache it will not be found.

This commit loads the normal msf environment, including lib/gemcache if
applicable, before starting msfgui.
Merge branch 'rapid7' into bturner-r7-msfgui-setup-env
Merge branch 'master' of github.com:rapid7/metasploit-framework into rapid7