Update to jspahrsummers/objc-build-scripts@a46f07e3fdf1cb0ec5f7f0fa5b2e8b870a8321d2

Also start using the workspace instead of the project file.
Prefer finding schemes in our project file

Since our workspace contains dependencies, we need to skip those on CI.
Merge pull request #1312 from ReactiveCocoa/update-build-scripts

Update build scripts