fix(node): remove node 6 and 8 support

.travis.yml

@@ -13,7 +13,6 @@ before_install:
 before_script:
 - git version
 script:
-- npm run lint
 - npm run test
 - npm run check-coverage
 after_success:

README.md

@@ -7,6 +7,11 @@ When you commit with Commitizen, you'll be prompted to fill out any required com
 
 ## Installing the command line tool
 
+Commitizen is currently tested against
+node 10 and 12 although it may work in
+older node. You should also have npm 6
+or greater.
+
 Installation is as simple as running the following command (if you see `EACCES` error, reading [fixing npm permissions](https://docs.npmjs.com/getting-started/fixing-npm-permissions) may help):
 
 ```

jobs/build.yml

@@ -11,12 +11,10 @@ jobs:
   strategy:
     maxParallel: 3
     matrix:
+      node-12:
+        node_version: ^12.6.0
       node-10:
         node_version: ^10.10.0
-      node-8:
-        node_version: ^8.12.0
-      node-6:
-        node_version: ^6.14.4
   steps:
   - task: NodeTool@0
     displayName: " Install Node.js"