Bump simple_oauth dependency version to ~> 0.2.0

No significant changes have been made to simple_oauth since 0.1.9, as you can easily see at https://github.com/laserlemon/simple_oauth/commits/v0.2.0

This change should have no effect other than conflict resolution with other gems that require simple_oauth 0.2.0 or higher. This became an issue for me because I want to use tumblife as well as the twitter gem which requires simple_oauth ~> 0.2.