Reduce Travis-CI build load

- Remove testing with --all-features, no features are supported
- Remove testing beta, it isn't useful and takes up valuable CI time

.travis.yml

@@ -35,20 +35,10 @@ matrix:
 
       script:
         - cargo test --verbose
-        - cargo test --verbose --all-features
 
     - language: rust
       rust: stable
       cache: cargo
 
       script:
-        - cargo test --verbose
-        - cargo test --verbose --all-features
-
-    - language: rust
-      rust: beta
-      cache: cargo
-
-      script:
-        - cargo test --verbose
-        - cargo test --verbose --all-features
+        - cargo test --verbose