CI updates incl. remove support for VS 2013.

* CI updates:

Travis: use cmake 3.7
Appveyor: Add .appveyor.yml config, test VS2013/VS2015/VS2017 x86/x64 Debug/Release.

* Drop VS2013 support from AppVeyor.

.travis.yml

@@ -93,7 +93,7 @@ install:
   - |
     if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
       if [[ -z "$(ls -A ${DEPS_DIR}/cmake/bin)" ]]; then
-        CMAKE_URL="https://cmake.org/files/v3.6/cmake-3.6.2-Linux-x86_64.tar.gz"
+        CMAKE_URL="https://cmake.org/files/v3.7/cmake-3.7.2-Linux-x86_64.tar.gz"
         mkdir -p cmake && travis_retry wget --no-check-certificate --quiet -O - "${CMAKE_URL}" | tar --strip-components=1 -xz -C cmake
       fi
       export PATH="${DEPS_DIR}/cmake/bin:${PATH}"

appveyor.yml

@@ -0,0 +1,48 @@
+shallow_clone: true
+
+platform:
+  - x86
+  - x64
+
+configuration:
+  - Debug
+  - Release
+
+image:
+  - Visual Studio 2015
+  - Visual Studio 2017
+
+cache:
+  - C:\cmake-3.7.2-win32-x86
+
+install:
+  - git clone --quiet --depth=1 https://github.com/Microsoft/unittest-cpp.git tests/unittest-cpp
+  - ps: |
+      if (![IO.File]::Exists("C:\cmake-3.7.2-win32-x86\bin\cmake.exe")) {
+        Start-FileDownload 'https://cmake.org/files/v3.7/cmake-3.7.2-win32-x86.zip'
+        7z x -y cmake-3.7.2-win32-x86.zip -oC:\
+      }
+      $env:PATH="C:\cmake-3.7.2-win32-x86\bin;$env:PATH"
+
+before_build:
+  - ps: |
+      mkdir build
+      cd build
+      if ("$env:APPVEYOR_JOB_NAME" -match "Image: Visual Studio 2015") {
+          $env:generator="Visual Studio 14 2015"
+      } else {
+          $env:generator="Visual Studio 15 2017"
+      }
+      if ($env:PLATFORM -eq "x64") {
+          $env:generator="$env:generator Win64"
+      }
+      echo generator="$env:generator"
+      cmake .. -G "$env:generator"
+
+build_script:
+  - cmake --build . --config %CONFIGURATION% -- /m /v:minimal
+
+test_script:
+  - ctest -j2
+
+deploy: off