ci(travis): stop testing in Node 4

BREAKING CHANGE: drop official support for Node 4.

There is no intentional change that breaks usage in Node 4, but as we
stop testing in this no longer supported Node version we recommend to
no update nock if you still rely on Node 4.

The reason why we decided to go ahead and make a breaking version
release is that it became increasingly harder to update dependencies
to the latest versions as they also drop support for Node 4.

.travis.yml

@@ -29,8 +29,6 @@ jobs:
       script: npm run unit
     - node_js: 6
       script: npm run unit
-    - node_js: 4
-      script: npm run unit
     - stage: release
       node_js: lts/*
       env: semantic-release