Updated TravisCI requirements for tape-run

.travis.yml

@@ -3,12 +3,11 @@ node_js:
   - '4'
   - '5'
   - '6'
-  - '7'
-  - '8'
+  - '7' # Node.js 7 is most stable version
 env:
   - CXX=g++-4.8
 before_script:
-  - npm install grunt-cli -g
+  - npm install grunt-cli -g # for "grunt build"
 script:
   - npm test
   - grunt build
@@ -18,3 +17,8 @@ addons:
       - ubuntu-toolchain-r-test
     packages:
       - g++-4.8
+      - xvfb # for tape-run
+install:
+  - export DISPLAY=':99.0' # for tape-run
+  - Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 & # for tape-run
+  - npm install # for tape-run