Update Homebrew formula

The Homebrew formula references a file that doesn't exist: https://github.com/tubbo/chruby-default-gems/archive/v1.0.1.tar.gz

These changes fix that.  This invocation works:

```
$ brew install https://raw.github.com/bronson/chruby-default-gems/patch-1/homebrew/chruby-default-gems.rb
```