Fixes rjsf-team#210 - Fixed Travis build failures (rjsf-team#212)

* Run Travis tests in node 6. * Update node engine requirement to >=6.

Author: n1k0

.travis.yml

@@ -2,7 +2,7 @@ sudo: false
 language:
 - node_js
 node_js:
-- "4"
+- "6"
 env:
 - ACTION=test
 - ACTION="run lint"

package.json

@@ -23,7 +23,7 @@
  "engineStrict": false,
  "engines": {
  "npm": "^2.14.7",
- "node": ">=4"
+ "node": ">=6"
  },
  "peerDependencies": {
  "react": "^0.14.0 || ^15.0.0"