drop support for Node.js v4 (#7755)

* drop support for Node.js v4; closes #7753

- remove version 4 from Travis build matrix
- update environment support doc
- update `CONTRIBUTING.md`
- update `engines` field of root `package.json`

Signed-off-by: Christopher Hiller <boneskull@boneskull.com>

* update engines for babylon

Signed-off-by: Christopher Hiller <boneskull@boneskull.com>

.travis.yml

@@ -10,7 +10,6 @@ node_js:
   # We test the latest version on circleci
   - '8'
   - '6'
-  - '4'
 
 env:
   global:

CONTRIBUTING.md

@@ -41,7 +41,7 @@ Feel free to check out the `#discussion`/`#development` channels on our [Slack](
 
 **Note:** Versions `< 5.1.10` can't be built.
 
-Babel is built for Node 4 and up but we develop using Node 8 and yarn. You can check this with `node -v`.
+Babel is built for Node 6 and up but we develop using Node 8 and yarn. You can check this with `node -v`.
 
 Make sure that Yarn is installed with version >= `0.28.0`.
 Installation instructions can be found here: https://yarnpkg.com/en/docs/install.

doc/design/compiler-environment-support.md

@@ -7,7 +7,7 @@
 The Babel compiler is **only** supported in these environments:
 
  - Modern browsers such as Chrome, Firefox, Safari, Edge etc.
- - Node.js 4 and upper versions
+ - Node.js 6 and newer versions
 
 ## Unsupported environments
 

package.json

@@ -68,7 +68,7 @@
     "webpack-stream": "^4.0.0"
   },
   "engines": {
-    "node": ">= 4.x <= 9.x",
+    "node": ">= 6.x <= 9.x",
     "npm": ">= 2.x <= 5.x",
     "yarn": ">=0.27.5 || >=1.0.0-20170811"
   },

packages/babylon/package.json

@@ -20,7 +20,7 @@
     "lib"
   ],
   "engines": {
-    "node": ">=4.2.0"
+    "node": ">=6.0.0"
   },
   "devDependencies": {
     "@babel/helper-fixtures": "7.0.0-beta.44",