travis: Allow Windows builds to fail

Travis CI builds for Windows are still experimental with no sign of
improvement for caching, pre-installed packages and other issues. Allow
it to fail such that Linux/macOS builds determine the build result.

Change-Id: I146e829bd89ab4f0e7bc9e1f1b89803c2294b75d
Reviewed-on: https://code.wireshark.org/review/32566
Reviewed-by: Dario Lombardo <lomato@gmail.com>

.travis.yml

@@ -76,6 +76,9 @@ matrix:
       cache:
         directories:
           - travis-cache
+  # Windows builds are experimental in Travis and are unreliable, do not force it.
+  allow_failures:
+    - os: windows
 before_install:
   - echo $TRAVIS_OS_NAME
   # macos