chore(ci): remove node 11 from CI

Commitlint doesn't work on it, and we can't upgrade commitlint since we support Node 4

Author: SimenB

.travis.yml

@@ -4,7 +4,6 @@ node_js:
 - 8
 - 6
 - 4
-- node
 before_install:
 - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.3.2
 - export PATH="$HOME/.yarn/bin:$PATH"