Updated travis.yml git depth 1

We don't do any git operation except include last commit name, so we only need last commit, that will make travis-ci test faster.

.travis.yml

@@ -3,6 +3,9 @@ language: node_js
 node_js:
   - "8"
 
+git:
+  depth: 1
+
 addons:
   apt:
     sources: