Fix failure to build when using Carthage

[kattrali]

Making the Bugsnag framework universal introduced an issue where the framework fails to build because of a framework linking issue where Cocoa.framework was included in the iOS build. This change resolves that issue, as well as a misleading issue where building for iOS would use OSX.xcworkspace, which is usable but potentially confusing.

There is one remaining issue. When building the framework, it will appear to build three times and then succeed, with output similar to this:

$ carthage update --platform ios
*** Cloning bugsnag-cocoa
*** Checking out bugsnag-cocoa at "d65082be79ead9908017e270f53a6d32fe41f5f3"
*** xcodebuild output can be found in /var/folders/5g/5fznsq314k94wlj5n0l2xph40000gn/T/carthage-xcodebuild.2rIVXB.log
*** Building scheme "Bugsnag" in Bugsnag.xcodeproj
*** Building scheme "Bugsnag" in Bugsnag.xcodeproj
*** Building scheme "Bugsnag" in Bugsnag.xcodeproj

This is an easily fixable issue by consolidating into a single Xcode project, however it has potential implications for people integrating bugsnag-cocoa as a git submodule or other vendoring solution, which could break if the project files are moved around. I've opted to wait on that change for now to determine the best way integrate everyone and write upgrade instructions.

Testing

• Tested manually against iOS / tvOS / and macOS projects using Carthage
• Added a new CI phase to build the repository with Carthage

Fixes #418