build: bump node from 6.x to 10.x

BREAKING CHANGE: drop support for old node versions

Author: gevalo1

.travis.yml

@@ -1,8 +1,17 @@
 language: node_js
+cache:
+  directories:
+    - $HOME/.npm
+notifications:
+  email: false
 node_js:
-  - "6.6"
+  - "10"
+  - "9"
 before_script:
   - npm install -g gulp karma-cli
   - npm install
 script: 
-  - npm test
+  - npm test
+branches:
+  except:
+    - /^v\d+\.\d+\.\d+$/