Fix Travis build (#4101)

Stop using gdb on osx - was breaking the build
Add clang 3.5 build as travis now defaults to 5.0
Fix unknown-warning-option errors on AppleClang
ProtoProxy: Use nullptr
UrlClientTest: add override to callback destructor
Update jsoncpp to use nullptr

tests/HTTP/UrlClientTest.cpp

@@ -19,7 +19,7 @@ public:
 	}
 
 
-	~cCallbacks()
+	virtual ~cCallbacks() override
 	{
 		LOGD("Deleting the cCallbacks instance at %p", reinterpret_cast<void *>(this));
 	}