Add Go 1.13 and Go 1.14 to build matrix (#1071)

Go 1.13 and 1.14 have been released since the last time we updated CI.
We don't expect any problems with them, but add them to the build matrix
for completeness.

Co-authored-by: Brandur <brandur@brandur.org>

.travis.yml

@@ -40,6 +40,8 @@ go:
   - "1.10.x"
   - "1.11.x"
   - "1.12.x"
+  - "1.13.x"
+  - "1.14.x"
   - tip
 
 language: go