Only build windows on master and allow failure

The Windows build is much slower than Linux, so we only build it on master
and make the build status not depend on it finishing up.

.travis.yml

@@ -16,11 +16,6 @@ build: &rust_build
 
 matrix:
   include:
-    - os: windows
-      before_script:
-        - dos2unix ./crates/ra_syntax/tests/data/parser/**/*.txt
-        - dos2unix ./crates/ra_syntax/tests/data/parser/**/*.rs
-      <<: *rust_build
     - os: linux
       <<: *rust_build
     # - language: rust
@@ -38,6 +33,12 @@ matrix:
         - cd editors/code && npm ci && npm run travis
 
   allow_failures:
+    - os: windows
+      if: branch = master
+      before_script:
+        - dos2unix ./crates/ra_syntax/tests/data/parser/**/*.txt
+        - dos2unix ./crates/ra_syntax/tests/data/parser/**/*.rs
+      <<: *rust_build
     - rust nightly
 
 branches: