Remove ESLint 6 from allowed failures in Travis

It's now supported

Author: sheepsteak

.travis.yml

@@ -68,7 +68,6 @@ matrix:
   # issues with typescript deps in this version intersection
   - node_js: '4'
     env: ESLINT_VERSION=4
-  - env: ESLINT_VERSION=6
 
 before_install:
   - 'nvm install-latest-npm'