Fix Implicitly Unwrapped nil value error in `XcodeSpec` using Optional NSString

<i>Related to #206</i>
<i>Refactor of #209</i>

When running `otool` in `XcodeSpec`, [the `value` of the `first` of the stream is implicitly unwrapped](https://github.com/Carthage/Carthage/blob/9368c474f195c91ac7d4b5c1ee3150bacec67f36/CarthageKitTests/XcodeSpec.swift#L74-77).

But, if `otool` hits an error, then `first()` returns a `.Failure`, whose `value()` returns `nil`, leading to `fatal error: unexpectedly found nil while unwrapping an Optional value`.
:ok_hand: