Update api.rb

I'm getting a syntax error with the colon.
Fetch Achievements game_id and gamertag switched

It kept throwing an error, so this is a futile attempt at trying
something new. I'm desperate.
gameid now titleid

Gem must be using v1.0 of the API, which had titleid. Thanks to
bscofield (my hero!).
Merge branch 'patch-1' of https://github.com/ashleybaxter/xbox_leaders into ashleybaxter-patch-1
Fix specs and misc. changes for api v2.0