Support Ruby 1.8

This should fix #150
Just looking over [Steve Klabnick's](https://github.com/steveklabnik/require_relative/blob/master/lib/require_relative.rb) version and he splits `caller.first` on `:`, since it returns a string like:

```ruby
"/home/fletch/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/irb/workspace.rb:86:in `eval'"
```

I guess `File.dirname` returns the same either way, though.