Revert "Update pinned golangci-lint version (#666)"

This reverts commit 015c076.
Goimports regression: golangci/golangci-lint#347
And gocritic recommending fixes in tip instead of released versions.

.travis.yml

@@ -12,7 +12,7 @@ git:
 env:
   global:
     - GOOS=linux GOARCH=amd64
-    - GOLANGCI_VERSION="v1.12.5"
+    - GOLANGCI_VERSION="v1.12.3"
 
 matrix:
   # It's ok if our code fails on unstable development versions of Go.