Remove lts/boron support

End-of-life scheduled for 2019-04-30: https://github.com/nodejs/Release

Dependencies requiring at least lts/carbon as of now:
- p-settle: https://github.com/sindresorhus/p-settle

Author: stovmascript

.travis.yml

@@ -4,5 +4,5 @@ os:
 language: node_js
 node_js:
   - "lts/*"
-  - "lts/boron"
+  - "lts/carbon"
   - "node"