chore(ci): drop Node.js 6 and add Node.js 12

BREAKING CHANGE:  This library will no longer be tested on Node.js 6 and as such will not be guaranteed to work on this version of Node.js. We support LTS versions of Node.js as they are specified here: https://github.com/nodejs/Release#release-schedule

.travis.yml

@@ -1,9 +1,9 @@
 language: node_js
 
 node_js:
+  - '12'
   - '10'
   - '8'
-  - '6'
 
 cache:
   directories:
@@ -12,9 +12,6 @@ cache:
 
 if: tag IS blank # do not build tags
 
-before_install:
-  - dpkg --compare-versions `npm -v` ge 6.0 || npm i -g npm@latest
-
 install:
   - if [[ "$TRAVIS_BRANCH" = "greenkeeper/"* ]]; then npm install; else npm ci; fi
 
@@ -35,4 +32,4 @@ jobs:
   include:
     - stage: publish
       node_js: '10'
-      script: npx semantic-release
+      script: semantic-release

package.json

@@ -14,7 +14,7 @@
     "url": "git+https://github.com/gurpreetatwal/incito.git"
   },
   "engines": {
-    "node": ">=4.0.0"
+    "node": ">=8.0.0"
   },
   "files": [
     "index.js",