switch to nvmrc for travis node version (#605)

.nvmrc

@@ -0,0 +1 @@
+11

.travis.yml

@@ -2,8 +2,6 @@ language: node_js
 before_install:
   - curl -o- -L https://yarnpkg.com/install.sh | bash
   - export PATH="$HOME/.yarn/bin:$PATH"
-node_js:
-  - 11
 cache: yarn
 script:
   - yarn lint