Removing support for Node 9

Node 9 is EOL and an upstream library no longer supports it. To avoid strange bugs,
we're explicitly removing support for it.

Author: weaverryan

.travis.yml

@@ -36,9 +36,6 @@ matrix:
     - os: linux
       node_js: "10"
       env: JOB_PART=test
-    - os: linux
-      node_js: "9"
-      env: JOB_PART=test
     - os: linux
       node_js: "8"
       env: JOB_PART=test

package.json

@@ -21,7 +21,7 @@
     "url": "https://github.com/symfony/webpack-encore/issues"
   },
   "engines": {
-    "node": ">=8.0.0"
+    "node": "8.* || >= 10.*"
   },
   "homepage": "https://github.com/symfony/webpack-encore",
   "dependencies": {