Only test released Go versions

Currently Go tip is not entirely happy with modules from Go 1.13,
but it's not something we care about until Go 1.14 is out.

Author: bobrik

.travis.yml

@@ -5,7 +5,6 @@ language: go
 
 go:
   - 1.x
-  - master
 
 before_install:
   # We're not interested in gopath provided by travis, default ~/go is good