Remove osx10.11 image name as it's no longer supported
Remove osx10.11 image name as it's no longer supported

The `osx_image: osx10.11` image name was not meant to be a long term supported thing, we've transition to just `xcodeX.x` based names, see https://docs.travis-ci.com/user/languages/objective-c/#Supported-Xcode-versions

Any builds on Travis will, unfortunately, fail, as we didn't realize this tag name was in use in the _wild_ anymore, until you merge in this change.

Thanks for understanding.
mikemcquaid/strap,mikemcquaid/strap,mikemcquaid/strap