Reorder test runs.

First do VS2015 since it is the most likely to break. Cygwin
is last because it is the slowest. Remove VS2017 because it
is being handled by Azure.

.appveyor.yml

@@ -4,18 +4,6 @@ os: Visual Studio 2015
 
 environment:
   matrix:
-    - arch: x86
-      compiler: msys2-mingw
-      backend: ninja
-
-    - arch: x64
-      compiler: msys2-mingw
-      backend: ninja
-
-    - arch: x64
-      compiler: cygwin
-      backend: ninja
-
     - arch: x86
       compiler: msvc2015
       backend: ninja
@@ -26,17 +14,17 @@ environment:
       backend: vs2015
       BOOST_ROOT: C:\Libraries\Boost_1_60_0
 
+    - arch: x86
+      compiler: msys2-mingw
+      backend: ninja
+
     - arch: x64
-      compiler: msvc2017
+      compiler: msys2-mingw
       backend: ninja
-      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
-      BOOST_ROOT: C:\Libraries\Boost_1_64_0
 
     - arch: x64
-      compiler: msvc2017
+      compiler: cygwin
-      backend: vs2017
+      backend: ninja
-      APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
-      BOOST_ROOT: C:\Libraries\Boost_1_64_0
 
 platform:
   - x64