Update instructions to install gem

This is what I get by using the `gem install heroku-pull` command:

![Error message](https://s3-us-west-2.amazonaws.com/droplr.storage/files/acc_184624/t1kx?AWSAccessKeyId=AKIAJSVQN3Z4K7MT5U2A&Expires=1386611920&Signature=t3CCSYpd9f4P7ochcdqEs%2FThjRE%3D&response-content-disposition=inline%3B%20filename%3DScreen%2520Shot%25202013-12-09%2520at%252010.56.26.png%3B%20filename%2A%3DUTF-8%2527%2527Screen%2520Shot%25202013-12-09%2520at%252010.56.26.png)

The correct, after searching in rubygems.org, is:

`gem install heroku_pull`


![Confirmation](https://s3-us-west-2.amazonaws.com/droplr.storage/files/acc_184624/6ymL?AWSAccessKeyId=AKIAJSVQN3Z4K7MT5U2A&Expires=1386612087&Signature=SZOI4NCZrvzkCY1MIf4Rdrs2%2Fso%3D&response-content-disposition=inline%3B%20filename%3DScreen%2520Shot%25202013-12-09%2520at%252011.01.18.png%3B%20filename%2A%3DUTF-8%2527%2527Screen%2520Shot%25202013-12-09%2520at%252011.01.18.png)