CombineExt 1.8.0

This version includes several bug fixes and new operators:

• Fixes some stubborn memory leaks in CurrentValueRelay (#137) and WithLatestFrom (#129) - Thanks @jasdeepsaini & @TTOzzi !
• Adds missing files to the project which prevented Carthage from building (#133) - Thanks @jasdeepsaini !
• Fix a replay synchronization issue in ReplaySubject (#138) - Thanks @mruston-heb !
• New flatMapFirst operator (#119) - Thanks @troupmar
• New prefix(while:behavior:) operator (#70) - Thanks @jasdev
• New mapToVoid operator (#129) - Thanks @danshevluk