travis: bump stable Go version to 1.12 and uncomment golangci-lint

Re-enable golangci-lint once golangci/golangci-lint#595 is resolved.

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 
 go:
-  - "1.11"
+  - "1.12"
   - tip
 
 notifications:
@@ -20,7 +20,8 @@ before_script:
   - go get golang.org/x/tools/cmd/goimports
   - go get golang.org/x/lint/golint
   - go get github.com/mattn/goveralls
-  - go get github.com/golangci/golangci-lint/cmd/golangci-lint
+  # TODO: re-enable when https://github.com/golangci/golangci-lint/issues/595 is resolved.
+  #- go get github.com/golangci/golangci-lint/cmd/golangci-lint
   - wget https://raw.githubusercontent.com/mewmew/ci/master/ci_checks.sh
   - chmod +x ci_checks.sh