Remove explicit faraday version and use config / errors compatible with < 0.8

This reverts 48bf42c47b8915d1484bd1419adb4261b9342298 and adds a switch based on whether Faraday options is a hash or options object. Additionally it uses the Faraday::Error class names which exist in Faraday 0.8 and 0.9.

This commit makes active-rest-client compatible with faraday versions 0.8 as well as 0.9+.