ci(travis): stop testing in Node 6, test in Node 12

.travis.yml

@@ -1,7 +1,5 @@
 language: node_js
-cache:
-  directories:
-    - ~/.npm
+cache: npm
 
 # Trigger a push build on master and greenkeeper branches + PRs build on every branches
 # Avoid double build on PRs (See https://github.com/travis-ci/travis-ci/issues/1147)
@@ -10,18 +8,12 @@ branches:
     - master
     - /^greenkeeper.*$/
 
-branches:
-  except:
-    - /^v\d+\.\d+\.\d+$/
-
 jobs:
   include:
     - stage: test
-      node_js: 6
-    - node_js: 8
-      install: npm ci
+      node_js: 8
     - node_js: 10
-      install: npm ci
+    - node_js: 12
     - node_js: lts/*
       script: npm run coverage:upload
     - stage: release