Travis CI: Upgrade to Node.js v6 (#386)

Also adjust Commander initialization to explicitly name our command,
which prevents a crash within commander within our CLI unit tests under Node.js 6.

Author: cvrebert

.travis.yml

@@ -1,7 +1,7 @@
 {
     "sudo": false,
     "language": "node_js",
-    "node_js": ["5"],
+    "node_js": ["6"],
     "before_install": [
         "travis_retry pip install -r test-infra/requirements.txt --user",
         "rvm install 2.2",

src/cli.js

@@ -4,13 +4,13 @@
 
 var Deferred = require('bluebird');
 var chalk = require('chalk');
-var program = require('commander');
+var commander = require('commander');
 var readFile = Deferred.promisify(require('fs').readFile);
 var glob = Deferred.promisify(require('glob'));
 var bootlint = require('./bootlint');
 
 module.exports = function () {
-    program
+    var program = (new commander.Command('bootlint'))
         .version(require('../package.json').version)
         .description('Lint the HTML of Bootstrap projects')
         .usage('[options] [files...]')