Make sure ~vagrant/.gem is owned by vagrant:vagrant

/etc/profile.d/gem-home.sh hard-codes '/home/vagrant'. If it happens to run as
root, the directory created is also owned by root, and not writable by vagrant.

Bug: 61285
Change-Id: Ib510b2e4d56574b25b3c7d1f3ead5585c50fc50d