Should use servicePath instead of basePath

[tjwebb]

using basePath returns urls prefixed with an extra slash. should use servicePath here: https://github.com/google/google-api-nodejs-client/blob/ce337ceba02a53dfde7e586c6ef1f58f47699553/lib/requests.js#L143

basePath appears deprecated according to: https://developers.google.com/discovery/v1/using#discovery-doc-apiproperties

[rakyll]

#109 and #116 fixed double slash problems.