fix(ci): use npx in travis and test only on node 6

Note: next version will require Node 8 & npm 5.2 & yarn 1.3
TAG: latest

Author: Charlike Mike Reagent

.travis.yml

@@ -10,20 +10,23 @@ cache:
     - node_modules
 
 node_js:
-  - '8'
+  # - '8'
   - '6'
 
 os: linux
 
 before_install:
   - curl -o- -L https://yarnpkg.com/install.sh | bash
   - export PATH=$HOME/.yarn/bin:$PATH
+  - yarn global add npx
+
+install: npx -p node@8 yarn install
 
 script: yarn test
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)
-  - yarn run release
+  - npx -p node@8 yarn run release
 
 branches:
   except: