Chore: Update travis build matrix (#37)

- Remove go versions prior to 1.13
- Add version 1.15

Author: Atte Kojo

Diff for: .travis.yml

@@ -1,17 +1,9 @@
 language: go
 
 go:
-  - 1.10.x
-  - 1.11.x
-  - 1.12.x
   - 1.13.x
   - 1.14.x
-
-matrix:
-  fast_finish: true
-  include:
-  - go: 1.13.x
-    env: GO111MODULE=on
+  - 1.15.x
 
 script:
   - go test -coverprofile=coverage.txt -covermode=atomic