[FIX] ImportError: cannot import name to_native_string

Actual version of requests-oauthlib uses a `requests` util available only on requests>=2.0.0

Until requests-oauthlib merges (this PR)[https://github.com/requests/requests-oauthlib/pull/151], is safer to ensure here the `requests` version needed

See: http://stackoverflow.com/a/24955127/798575
And: https://github.com/requests/requests-oauthlib/pull/151