fix(travis-ci): Remove older versions of go (#516)

Remove older versions of go from travis config due to the testing
dependency testify requiring errors.Is which is only available in go
1.13 and above.

Also switch to master from tip which is the new standard for travis
on golang..

.travis.yml

@@ -3,17 +3,13 @@ services:
   - redis-server
 
 go:
-  - 1.9.x
-  - 1.10.x
-  - 1.11.x
-  - 1.12.x
   - 1.13.x
   - 1.14.x
-  - tip
+  - master
 
 matrix:
   allow_failures:
-    - go: tip
+    - go: master
 
 script:
   - go get -t -v ./...