build: run macOS build only in bors branches

Clippy is portable on *nix OSes. It is really rare to see
clippy broken on either macOS and Linux. So let's test it
on bors branches only to save CI time.

Author: tesuji

.travis.yml

@@ -47,8 +47,9 @@ matrix:
   fast_finish: true
   include:
     # Builds that are executed for every PR
-    - os: osx # run base tests on both platforms
     - os: linux
+    - os: osx
+      if: branch IN (auto, try)
     - os: windows
       env: CARGO_INCREMENTAL=0 OS_WINDOWS=true