.travis += go1.12, go1.13, go1.14 and go1.15

Go1.14 and Go1.15 are the current stable Go releases.
Go1.12 and Go1.14 fill in the gap in testing coverage in between -go1.11
and current go releases.

Maybe we can eventually drop older go release from travis coverage.

.travis.yml

@@ -6,4 +6,8 @@ go:
   - 1.9
   - "1.10"
   - "1.11"
+  - "1.12"
+  - "1.13"
+  - "1.14"
+  - "1.15"
   - tip