added setVersion for setting graph api version

Took me long enough! Apologies for the delay.

@xpepermint commented [here](https://github.com/criso/fbgraph/issues/54#issuecomment-57127331) regarding also adding the version to the OAuth dialog URLs, which would probably work, but it's not officially recommended or documented in [Facebook's docs](https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow/v2.1), so I didn't add it.

Didn't try getting the tests to work as you mentioned they were a little flaky.