Change the Travis config to allow failures in rust beta.

Rustc beta and nightly crash when compiling Cretonne. I've filed
https://github.com/rust-lang/rust/issues/49528 to track this upstream.
For now, add Beta to the allow-failures list to temporarily work
around this.

.travis.yml

@@ -5,7 +5,9 @@ rust:
     - nightly
 matrix:
     allow_failures:
-        - rust: nightly
+        - rust:
+            - beta
+            - nightly
 dist: trusty
 sudo: false
 addons: