Fix cached_location, new repo and dependency for libstrophe.

I kept getting this error when trying to use this formula:

```ruby
Error: undefined method `cached_location' for nil:NilClass
```

While trying to debug the install I noticed that the repo for libstrophe changed and a dependency on `check` was added.  After making these changes everything worked.  

This pull request incorporates all of the changes I had to make.

Thanks for the formula!