chore: update travis build file (#17)

* remove double-build on PRs
* build on latest & LTS node

Author: achingbrain

.travis.yml

@@ -1,14 +1,20 @@
 language: node_js
 cache: npm
+
+branches:
+  only:
+  - master
+  - /^release\/.*$/
+
+node_js:
+  - 'lts/*'
+  - 'node'
+
 stages:
   - check
   - test
   - cov
 
-node_js:
-  - '10'
-  - '12'
-
 os:
   - linux
   - osx
@@ -58,4 +64,3 @@ jobs:
 
 notifications:
   email: false
-