chore(travis): add build config for 'stable' tests

.travis.yml

@@ -9,6 +9,8 @@ matrix:
           env: FEATURES="--features nightly" BENCH=true
           sudo: false
         - rust: beta
+          sudo: false
+        - rust: stable
           sudo: true
 
 cache:
@@ -18,15 +20,15 @@ cache:
 script: ./.travis.sh
 
 after_success: |
-    [ $TRAVIS_RUST_VERSION = beta ] &&
+    [ $TRAVIS_RUST_VERSION = stable ] &&
     sudo apt-get install libcurl4-openssl-dev libelf-dev libdw-dev &&
     wget https://github.com/SimonKagstrom/kcov/archive/master.tar.gz &&
     tar xzf master.tar.gz && mkdir kcov-master/build && cd kcov-master/build && cmake .. && make &&
     sudo make install && cd ../.. &&
     kcov --coveralls-id=$TRAVIS_JOB_ID --exclude-pattern=/.cargo target/kcov target/debug/hyper-*
     [ $TRAVIS_BRANCH = master ] &&
     [ $TRAVIS_PULL_REQUEST = false ] &&
-    [ $TRAVIS_RUST_VERSION = beta ] &&
+    [ $TRAVIS_RUST_VERSION = stable ] &&
     cargo doc  &&
     echo '<meta http-equiv=refresh content=0;url=hyper/index.html>' > target/doc/index.html &&
     pip install --user ghp-import &&