Add --no-ri --no-rdoc to "gem i" as well

This matches the flags used in line 42:

   gem update rails --no-ri --no-rdoc

Without these flags, OS X 10.9 users could run into:

    https://github.com/rails/rails/issues/11814

where installation seems to hang.

( Also sent to upstream as https://github.com/railsgirls/railsgirls.github.com/pull/162 )