Catch up with the fixes.

Fix linter.

What's done:
  * change linter from deprecated golint to revive
  * exclude SA5011 (deprecated strings.Title() call) -- the string
    passed to it is guaranteed to be ASCII
  * move nil-pointer check before first pointer dereferece in
    cvefeed/cvecache.go
  * proper declare-initialze syntax in
    providers/redhat/package_feed_test.go:feedSummary()
  * replace if strings.HasSuffix(..) { // manually trim suffix } with
    strings.TrimSuffix() in rpm/parse.go