Move Travis Windows build to allowed failures

Until the remaining issues are fixed. This also enabled `fast_finish`.
It will finish even if the windows build is still running.

.travis.yml

@@ -52,6 +52,9 @@ matrix:
     - env: INTEGRATION=serde-rs/serde
     - env: INTEGRATION=Geal/nom
     - env: INTEGRATION=hyperium/hyper
+  allow_failures:
+  - os: windows
+    env: BASE_TEST=true
 # prevent these jobs with default env vars
   exclude:
     - os: linux