Fix 64-bit iOS unit tests

Under 64-bit iOS, the `BOOL` type has changed from `signed char` to `bool` but NSNumber did not get the memo. So `[@NO objCType]` (which is equivalent to `[[NSNumber numberWithBool:NO] objCType]`) returns "c" (_C_CHR) instead of "B" (_C_BOOL). OCMock uses this information to check that the mock return type is compatible with the expected return type. Using the OCMOCK_VALUE macro solves the problem since it explicitly encodes the `BOOL` type.