Bumped vagrant version for net-ssh conflict

When attempting to use vagrant-rake in a Rails project which depends on
Capistrano, there was a conflict between vagrant-rake ~> vagrant 0.6
and vagrant 0.6 -~ net-ssh 2.0.23 and capistrano's net-ssh 2.6.7.

Bumped vagrant version to allow vagrant > 0.6 to resolve this.