chore: update travis config

Author: gregberge

.travis.yml

@@ -1,17 +1,10 @@
 language: node_js
 
 node_js:
-  - 6
   - 8
-
-before_install:
-  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
-  - export PATH="$HOME/.yarn/bin:$PATH"
+  - 10
 
 notifications:
   email: false
 
-cache:
-  yarn: true
-  directories:
-    - "node_modules"
+cache: yarn