update travis to use golangci-lint binary instead of building from source

Author: atomicptr

.travis.yml

@@ -11,7 +11,7 @@ go:
   - '1.15'
 before_script:
   - go get -u github.com/mattn/goveralls
-  - go get -u github.com/golangci/golangci-lint/cmd/golangci-lint
+  - curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(go env GOPATH)/bin v1.35.2
 before_install:
   - openssl aes-256-cbc -K $encrypted_7ebe6f038385_key -iv $encrypted_7ebe6f038385_iv
     -in snap.login.enc -out snap.login -d