update golangci-lint to v1.30.0 and add go 1.15.x to build matrix

Author: simukti

.travis.yml

@@ -5,6 +5,7 @@ go:
   - 1.12.x
   - 1.13.x
   - 1.14.x
+  - 1.15.x
   - master
 
 matrix:
@@ -30,7 +31,7 @@ before_install:
 # golangci.com will be closed on 2020-04-15
 # https://medium.com/golangci/golangci-com-is-closing-d1fc1bd30e0e
 before_script:
-  - curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/v1.23.8/install.sh | sh -s -- -b $GOPATH/bin v1.23.8
+  - curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(go env GOPATH)/bin v1.30.0
 
 script:
   - go vet