Change steam strategy require to be truly relative

This change makes it explicit that the require is loaded relatively. As it is, the `lib/omniauth/strategies/steam.rb` in the `omniauth-openid` gem seems to be taking precedence (the [changes you made](https://github.com/intridea/omniauth-openid/commit/c8d85def5ffc3953eb5f0407d880da9412ccc553) have not been mirrored in `omniauth-openid`'s repository), and is therefore breaking `omniauth-steam` when these two gems are implemented together. Details [here](https://github.com/intridea/omniauth-openid/issues/22).