Require Node >= 6 (drop support for Node 4)

Node 4 is no longer supported and dependencies are starting to fail when
running on Node 4 (e.g. nyc and yargs).  Without known use cases, it's
not worth supporting Node < 6 any longer.

Signed-off-by: Kevin Locke <[email protected]>

Author: kevinoid

.travis.yml

@@ -4,7 +4,7 @@ language: node_js
 # Update engines.node in package.json if removing old versions.
 node_js:
   - node
-  - 4
+  - 6
 
 # If the package has OS-dependent features, uncomment this.
 #os:

appveyor.yml

@@ -4,10 +4,8 @@
 # Update engines.node in package.json if removing old versions.
 environment:
   matrix:
-    - nodejs_version: "4"
-      # npm 6.0.0 fails on Node 4
-      # Note: Can't use carat version https://github.com/npm/npm/issues/17297
-      npm_version: ">=5 <6"
+    - nodejs_version: "6"
+      npm_version: "latest"
       # Empty version installs most recent version available
     - nodejs_version: ""
       npm_version: "latest"

package.json

@@ -54,7 +54,7 @@
     "rimraf": "^2.2.0"
   },
   "engines": {
-    "node": ">=4",
+    "node": ">=6",
     "npm": ">=1.3.7"
   },
   "yargs": {