Fail on implicit convert from BOOL on ObjC in folly::dynamic

Fail on implicit convert from BOOL on ObjC in folly::dynamic

Summary: On some platforms (iPhone 5C at least), ObjC/Objc++ BOOL is really signed char.  There is code which expects this to be a boolean when converted to dynamic (and then to JSON and into JS), but the old code treated it as a number.  This makes such code (like [mobileConfig getBool:]) fail to compile, so the developer needs to resolve the ambiguity one way or the other.

Reviewed By: yfeldblum

Differential Revision: D4648133

fbshipit-source-id: 76ece7803a1e966dca08bdb857af7990035544a0