travis: drop Go 1.13 builders as it's not supported any more

.travis.yml

@@ -24,16 +24,6 @@ jobs:
       script:
         - go run build/ci.go lint
 
-    - stage: build
-      os: linux
-      dist: xenial
-      go: 1.13.x
-      env:
-        - GO111MODULE=on
-      script:
-        - go run build/ci.go install
-        - go run build/ci.go test -coverage $TEST_PACKAGES
-
     - stage: build
       os: linux
       dist: xenial