fix: set node@4 as minimum (#32)

Also, don't test node@5, it's EOL

Author: SimenB

.babelrc

@@ -8,7 +8,7 @@
       "env",
       {
         "targets": {
-          "node": 5
+          "node": 4
         }
       }
     ]

.travis.yml

@@ -2,7 +2,7 @@ language: node_js
 node_js:
   - 7
   - 6
-  - 5
+  - 4
 before_install:
   - npm config set depth 0
   - npm install --global npm@latest

package.json

@@ -24,7 +24,7 @@
     "semantic-release": "^6.3.2"
   },
   "engines": {
-    "node": ">=5"
+    "node": ">=4"
   },
   "keywords": [
     "eslint",