Allow rust: beta to fail (for now):

wahn · wahn · commit 7a9e007daa4b · 2017-05-15T13:42:01.000+02:00
See rust-lang/rust#41944
diff --git a/.travis.yml b/.travis.yml
@@ -5,4 +5,5 @@ rust:
   - nightly
 matrix:
   allow_failures:
+    - rust: beta
     - rust: nightly
