Update travis.yml and appveyor.yml

.travis.yml

@@ -1,6 +1,7 @@
 dist: xenial
 language: minimal
 compiler: gcc
+os: linux
 addons:
   apt:
     sources:
@@ -32,10 +33,8 @@ addons:
 matrix:
   include:
     - name: Build GCC
-      os: linux
 
     - name: Build Clang
-      os: linux
       env: CC=clang CXX=clang++
 
     - name: Build OSX

appveyor.yml

@@ -4,7 +4,8 @@ image: Visual Studio 2017
 environment:
   matrix:
     - TARGET: msvc
-    - TARGET: mingw
+    # mingw currently causes unnecessary compilation errors due to appveyor-internal problems
+    # - TARGET: mingw
 
 install:
     - IF "%TARGET%"=="mingw" (choco install InnoSetup)