updated cmake min version

.travis.yml

@@ -37,7 +37,7 @@ matrix:
     compiler: clang
     env: COMPILER=clang++
   - os: osx
-    osx_image: xcode9.1
+    osx_image: xcode9.4
     compiler: clang
     env: COMPILER=clang++
   - os: linux

CMakeLists.txt

@@ -2,7 +2,7 @@
 # EnTT
 #
 
-cmake_minimum_required(VERSION 3.2)
+cmake_minimum_required(VERSION 3.7.2)
 
 #
 # Building in-tree is not allowed (we take care of your craziness).