update CI to use Go 1.13.3

This is the latest release. Updating is useful to ensure that we have
all of the latest fixes and enhancements.

Author: pohly

travis.yml

@@ -6,7 +6,7 @@ git:
   depth: false
 matrix:
   include:
-  - go: 1.12.4
+  - go: 1.13.3
 before_script:
 - mkdir -p bin
 - wget https://github.com/golang/dep/releases/download/v0.5.1/dep-linux-amd64 -O bin/dep