fix: bumped cross-spawn to version 7.0.0 (#211)

* bumped cross-spawn to version 7.0.0 and added a warning about node.js 7 and lower

* package.json, engines node : ">=8.0"

BREAKING CHANGE: Drop support for Node.js < 7.

README.md

@@ -54,6 +54,9 @@ npm install --save-dev cross-env
 > WARNING! Make sure that when you're installing packages that you spell things
 > correctly to avoid [mistakenly installing malware][malware]
 
+> NOTE : Version 6 of cross-env only supports Node.js 8 and higher, to use it on Node.js 7 or lower install version 5
+> ```npm install --save-dev cross-env@5 ```
+
 ## Usage
 
 I use this in my npm scripts:

package.json

@@ -9,7 +9,7 @@
     "cross-env-shell": "dist/bin/cross-env-shell.js"
   },
   "engines": {
-    "node": ">=4.0"
+    "node": ">=8.0"
   },
   "scripts": {
     "add-contributor": "kcd-scripts contributors add",
@@ -25,7 +25,7 @@
   "author": "Kent C. Dodds <kent@doddsfamily.us> (http://kentcdodds.com/)",
   "license": "MIT",
   "dependencies": {
-    "cross-spawn": "^6.0.5"
+    "cross-spawn": "^7.0.0"
   },
   "devDependencies": {
     "kcd-scripts": "^0.3.4"