Update & simplify vagrant instructions.
Update & simplify vagrant instructions.

The gem installer is deprecated, direct people to the real installer.
No longer need to require XCode and RVM installs, just install vagrant
and go. Password prompt during boot only happens on windows, not OS X
and Linux hosts.

tadp/learnswift,natefinch/discourse,tadp/learnswift,natefinch/discourse,tadp/learnswift