fix(node): remove node 6 and 8 support (#649)

This removes node 6 and 8 support. This is a free project and our builds are getting very long so we
have to be a bit careful about what versions we test on. To be clear, this still works on Node 6 and
8 for now but we won't be maintaining that guarantee into the future so please work towards
updating. This also removes eslint, likely in favor of prettier at some future date.

BREAKING CHANGE: Removal of two major node versions, 6 and 8 from testing scheme

.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"