Fix typo in guide post links
Add rbenv-gem-rehash to the install guide

We ran into problems when trying to verify the installation of the rails
stack via a simple `rails new railsgirls` since rbenv does not automatically
add shims for newly installed gems. The rbenv-gem-rehash plugin add this
behavior an fixes the problem.
Merge pull request #169 from skippednote/master

Fix typo in guide post links
Merge pull request #171 from croaker/master

Add rbenv-gem-rehash to the install guide
Merge pull request #170 from FloorD/master

update app guide with JS related tip