Add Node.js 7 to package.json and .travis.yml

Added Node.js 7 to package.json. This way yarn will not complain when
Node.js 7 is used. Tests still pass and the code itself also works
on Node.js 7.

Configure Travis CI to run the build on Node.js 7 too, in order to
verify correctness.

.travis.yml

@@ -3,3 +3,4 @@ language: node_js
 node_js:
   - "4"
   - "6"
+  - "7"

package.json

@@ -3,7 +3,7 @@
   "version": "3.1.0",
   "description": "Current context for LoopBack applications, based on cls-hooked",
   "engines": {
-    "node": "^4.5 || ^5.10 || ^6.0"
+    "node": "^4.5 || ^5.10 || ^6.0 || ^7.0"
   },
   "keywords": [
     "StrongLoop",