Mac OS code needs fixing

[Ybalrid]

Apparently, current code attempt to instantiate an abstract class in the Cocoa code.

https://travis-ci.org/Ybalrid/OIS/jobs/357121093#L263

log:

[ 54%] Building CXX object CMakeFiles/OIS.dir/src/mac/MacKeyboard.cpp.o
[ 59%] Building CXX object CMakeFiles/OIS.dir/src/mac/MacMouse.cpp.o
[ 63%] Building CXX object CMakeFiles/OIS.dir/src/mac/CocoaInputManager.mm.o
/Users/travis/build/Ybalrid/OIS/src/mac/CocoaInputManager.mm:161:27: error: allocating an object of abstract class type 'OIS::CocoaKeyboard'
                obj = new CocoaKeyboard(this, bufferMode, mUseRepeat);
                          ^
/Users/travis/build/Ybalrid/OIS/includes/OISKeyboard.h:295:24: note: unimplemented pure virtual method 'getAsKeyCode' in 'CocoaKeyboard'
                virtual OIS::KeyCode getAsKeyCode(std::string str) = 0;
                                     ^
1 error generated.

[Ybalrid]

Fixed (workaround) in f974c02266ee2ba4707a1f9ec13a73f5fc8a0260