Removes prettier-eslint, replaces yarn with npm, upgrades to node 8.

Author: ctrlplusb

.eslintrc

@@ -1 +1 @@
-6.10.0
+8

.nvmrc

@@ -1,13 +1,13 @@
 sudo: false
 language: node_js
 cache:
-  yarn: true
+  npm: true
   directories:
     - node_modules
 node_js:
-- '6'
+- '8'
 script:
-- yarn run test
+- npm run test
 after_success:
 # Deploy code coverage report to codecov.io
-- yarn run test:coverage:deploy
+- npm run test:coverage:deploy