Travis: Windows: target Visual Studio 2017

Only the Visual Studio 2017 toolset is currently preinstalled on Travis.
Use this, instead of our solution's default which is VS 2010.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/.travis.yml b/.travis.yml
index 057875a..ac997b0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -41,7 +41,7 @@
     - name: Windows
       os: windows
       script:
-        - scripts/windows_msbuild.bat
+        - scripts/windows_msbuild.bat v141 # Visual Studio 2017
 
 after_failure:
 - tests/scripts/travis-log-failure.sh