Make beta required to pass on travis again. (#67)

SirVer · wally-the-cartographer · commit bb73289523a3 · 2018-01-15T10:43:36.000+01:00
Upstream bug rust-lang/rust#47175 is fixed, so we enable beta again as required for passing.
diff --git a/.travis.yml b/.travis.yml
@@ -8,7 +8,6 @@ rust:
   - nightly
 matrix:
   allow_failures:
-    - rust: beta
     - rust: nightly
 
 addons:
