travis: ignore failed `brew upgrade npm`, update xcode
travis: ignore failed `brew upgrade npm`, update xcode

PR-URL: https://github.com/nodejs/node-gyp/pull/1932
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>

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