Update travis config to test on stable (#273)

.travis.yml

@@ -3,8 +3,7 @@ dist: trusty
 language: rust
 services: docker
 sudo: required
-#rust: stable
-rust: beta # we need 1.36, which is still beta
+rust: stable
 
 env:
   global:
@@ -20,7 +19,7 @@ matrix:
     #
     # This job will also build and deploy the docs to gh-pages.
     - env: TARGET=x86_64-unknown-linux-gnu
-      #rust: 1.36.0 (not stable yet)
+      rust: 1.36.0
       after_success:
         - |
             pip install 'travis-cargo<0.2' --user &&
@@ -44,7 +43,7 @@ matrix:
     - env: EXTRA_ARGS="--features either"
 
     # WASM support
-    - rust: beta
+    - rust: stable
       script:
         - rustup target add wasm32-unknown-unknown
         - cargo build --target=wasm32-unknown-unknown