gyp: fix TypeError in XcodeVersion()
gyp: fix TypeError in XcodeVersion()

PR-URL: https://github.com/nodejs/node-gyp/pull/1939
Reviewed-By: Jiawen Geng <8f5a7089e1809fdab8e8d97ec73a77a8b0770d74@gmail.com>
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>
Reviewed-By: Sam Roberts <266bbf644f3674ab209c4709d090256bac1c7bb7@gmail.com>

nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodejs/node-gyp