Minetest for C++11 (CMakeLists + Travis)

* Move GCC to GCC 6 & GCC 7
* Move Clang to Clang 3.6 & Clang 4.0
* LINT moves from Clang 3.9 to Clang 4.0
* Move XCode 7.3 to 8.0
* Use more travis tricks to install compilers instead of adding complexity to our build script
* Clang format fixes on checked files (compat Cpp11 instead of Cpp03)
* Mingw GCC update from 4.8.4 to 5.3 (Ubuntu Xenial)
* Drop mingw cmake generated files and add them to gitignore

src/environment.h

@@ -79,6 +79,7 @@ public:
 	u32 m_added_objects;
 
 	IGameDef *getGameDef() { return m_gamedef; }
+
 protected:
 	GenericAtomic<float> m_time_of_day_speed;