WIP: Temporarily drop stable on CI until async/await is stable

TODO: Delete this commit once rust 1.39.0 is released

Author: jtgeibel

.travis.yml

@@ -44,7 +44,7 @@ matrix:
     - rust: nightly
   include:
     - name: Backend
-      rust: stable
+      rust: beta
       before_install:
         - rustup component add rustfmt-preview
         - rustup component add clippy-preview
@@ -63,9 +63,6 @@ matrix:
         - npm run lint:js
         - npm run lint:deps
         - npm test
-    - rust: beta
-      script:
-        - cargo test
     - rust: nightly
       script:
         - cargo test