Updated post_install_message in Gemspec file

Added one \n at the end of post_install_message in gemspec file, since cursor for next command appears just next to the end of the message.

Attached Screenshot.

![twitter_bootstrap_rails](https://f.cloud.github.com/assets/1220255/653805/200efa26-d4e3-11e2-865d-14887ac4d030.png)