Use matrix(jobs) in .travis.yml

.travis.yml

@@ -1,12 +1,21 @@
 language: node_js
 dist: trusty
+cache: yarn
+env:
+  global:
+    - CXX=g++-4.8
+
+jobs:
+  include:
+    - env: task=npm-test
       node_js:
         - 6
-  - 7
-env:
-  - CXX=g++-4.8
-cache: yarn
-
+      before_install:
+        - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.1.0
+        - export PATH="$HOME/.yarn/bin:$PATH"
+    - env: task=npm-test
+      node_js:
+        - 7
       before_install:
         - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.1.0
         - export PATH="$HOME/.yarn/bin:$PATH"