Breaking: Drop support for Node.js v6 (refs #138)

The new processor API first shipped in ESLint v6, which requires at
least Node.js v8.10.0.

Since Node.js v8 left LTS maintenance two months ago, I considered
dropping support for it as well and aligning with the supported engines
in ESLint v7, but I don't see a compelling reason not to support ESLint
v6.

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [6.x, 8.x, 9.x, 10.x, 12.x]
+        node-version: ["8.10.0", 8.x, 10.x, 12.x, 13.x]
     steps:
     - name: Checkout
       uses: actions/checkout@v1

.travis.yml

@@ -1,10 +1,10 @@
 language: node_js
 node_js:
-  - "6"
+  - "8.10.0"
   - "8"
-  - "9"
   - "10"
   - "12"
+  - "13"
 sudo: false
 notifications:
   email: false

package.json

@@ -50,6 +50,6 @@
     "unified": "^6.1.2"
   },
   "engines": {
-    "node": "^6.14.0 || ^8.10.0 || >=9.10.0"
+    "node": "^8.10.0 || ^10.12.0 || >= 12.0.0"
   }
 }