Capitalize git repo URL (it is case sensitive)

This PR corrects the capitalization of the git repo URL in package.json.

```
$ git clone git://github.com/learnboost/stylus
Cloning into 'stylus'...
fatal: remote error: 
  Repository not found.
$ git clone git://github.com/LearnBoost/stylus
Cloning into 'stylus'...
remote: Counting objects: 15956, done.
remote: Compressing objects: 100% (8640/8640), done.
remote: Total 15956 (delta 7641), reused 15269 (delta 6982)
Receiving objects: 100% (15956/15956), 4.63 MiB | 394 KiB/s, done.
Resolving deltas: 100% (7641/7641), done.
```