Update CI to test against stable Go versions

Author: theckman

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 go:
-  - 1.10.x
-  - 1.11.x
+  - 1.14.x
+  - 1.15.x
 script: go test -v -check.vv -race ./...
 sudo: false
 notifications:

appveyor.yml

@@ -7,7 +7,7 @@ clone_folder: 'c:\gopath\src\github.com\gofrs\flock'
 
 environment:
   GOPATH: 'c:\gopath'
-  GOVERSION: '1.11'
+  GOVERSION: '1.15'
 
 init:
   - git config --global core.autocrlf input