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

@@ -41,7 +41,7 @@ addons:
 matrix:
   fast_finish: true
   allow_failures:
-    - rust: nightly
+    - rust: stable
   include:
     - name: Backend
       rust: stable
@@ -64,7 +64,12 @@ matrix:
         - npm run lint:deps
         - npm test
     - rust: beta
+      before_install:
+        - rustup component add rustfmt-preview
+        - rustup component add clippy-preview
       script:
+        - cargo fmt -- --check
+        - cargo clippy --all-targets --all-features --all
         - cargo test
     - rust: nightly
       script: