appveyor: explicitly specify build images

AppVeyor currently does provide three standard build worker images with
VS2013, VS2015 and VS2017. Right now, we are using the implicitly, which
is the VS2015 one. We want to be more explicit about this, so that we
can easily switch build images based on the job. So starting from this
commit, we explicitly set the `APPVEYOR_BUILD_WORKER_IMAGE` variable per
job, which enables us to choose different images.

To be able to test a wider range of build configurations, this commit
also switches the jobs for VC2010 over to use the older, VS2013 based
images. As the next commit will introduce two new jobs for building with
VS2015, we have then covered both build environments.

Also, let us be a bit more explicit regarding the CMake generator.
Instead of only saying "Visual Studio 10", use the more descriptive
value "Visual Studio 10 2010" to at least avoid some confusion
surrounding the versioning scheme of Visual Studio.

appveyor.yml

@@ -9,13 +9,17 @@ environment:
   GITTEST_INVASIVE_FS_SIZE: 1
 
   matrix:
-  - GENERATOR: "Visual Studio 11"
+  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2013
+    GENERATOR: "Visual Studio 10 2010"
     ARCH: 32
-  - GENERATOR: "Visual Studio 11 Win64"
+  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2013
+    GENERATOR: "Visual Studio 10 2010 Win64"
     ARCH: 64
-  - GENERATOR: "MSYS Makefiles"
+  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+    GENERATOR: "MSYS Makefiles"
     ARCH: i686 # this is for 32-bit MinGW-w64
-  - GENERATOR: "MSYS Makefiles"
+  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+    GENERATOR: "MSYS Makefiles"
     ARCH: 64
 cache:
 - i686-4.9.2-release-win32-sjlj-rt_v3-rev1.7z