Workaround for Swift compiler optimizer bug

Figured this out in the process of trying to pinpoint the crash to submit to the [swift-compiler-crashes](https://github.com/practicalswift/swift-compiler-crashes/pull/38) project.

- Set release mode optimization level back to "Fastest [-O]" for both iOS & OS X
- Tests pass
- In ref to:
    - https://github.com/DaveWoodCom/XCGLogger/issues/10
    - https://github.com/DaveWoodCom/XCGLogger/issues/17
    - https://github.com/DaveWoodCom/XCGLogger/issues/26