Try mingw testing again.

appveyor.yml

@@ -60,12 +60,12 @@ environment:
     CXXFLAGS: /std:c++latest
     platform: 64
     APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
-#  - TOOLSET: mingw-5
-#    platform: 32
-#    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
-#  - TOOLSET: mingw64-6
-#    platform: 64
-#    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+  - TOOLSET: mingw-5
+    platform: 32
+    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+  - TOOLSET: mingw64-6
+    platform: 64
+    APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
 
 # We can also set up configurations for testing which map to
 # the b2 build variants.