Build: Set the minimum required compiler version to GCC 6.3

* This is the current GCC version of debian 9

(cherry picked from commit 5a9ccae3e1)

.travis.yml

@@ -10,7 +10,10 @@ git:
 
 addons:
   apt:
+    sources:
+      - ubuntu-toolchain-r-test
     packages:
+      - g++-6
       - libboost1.55-dev
       - libboost-filesystem1.55-dev
       - libboost-iostreams1.55-dev

cmake/compiler/gcc/settings.cmake

@@ -1,7 +1,7 @@
 # Set build-directive (used in core to tell which buildtype we used)
 add_definitions(-D_BUILD_DIRECTIVE='"${CMAKE_BUILD_TYPE}"')
 
-set(GCC_EXPECTED_VERSION 4.9.0)
+set(GCC_EXPECTED_VERSION 6.3.0)
 
 if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS GCC_EXPECTED_VERSION)
   message(FATAL_ERROR "GCC: TrinityCore requires version ${GCC_EXPECTED_VERSION} to build but found ${CMAKE_CXX_COMPILER_VERSION}")