Made use of Travis CI's Windows support.

At least for now there seems to be no point in trying to build the libraries (and run the tests) on Windows. Rather we only want to check that the MSVC solution is still valid.

.travis.yml

@@ -1,5 +1,9 @@
-language:
-  - c
+language: c
+
+jobs:
+  include:
+
+  - os: linux
     install:
       - sudo apt-get update
       - sudo apt-get install linuxdoc-tools linuxdoc-tools-info binutils-mingw-w64-i686 gcc-mingw-w64-i686 sshpass
@@ -14,3 +18,7 @@ script:
       - make doc zip
     after_success:
       - make -f Makefile.travis
+
+  - os: windows
+    script:
+      - MSBuild.exe scr/cc65.sln