Allow failures on beta toolchain

Some dependencies don't guarantee beta compatibility making it difficult
for racerd to do so. In this case, the latest version of clap (racer
dep) is broken.

.travis.yml

@@ -14,6 +14,7 @@ rust:
 matrix:
   allow_failures:
     - rust: nightly
+    - rust: beta
 
 script:
   - cargo test