Updating to work with GuzzleHttp 5.0

- updating composer.json to allow for both 4.0 and 5.0
- removed fluent interface usage in OAuth1
- updating OAuth1Test to work with 5.0. Moved the `GuzzleHttp\Adapter\Transaction` to `GuzzleHttp\Transaction`. *note:* this is bc incompatible with 4.0

Signed-off-by: RJ Garcia <rj@bighead.net>