Limit RuboCop Encoding cop to only warn when needed

This was printing a bunch of unnecessary warnings for the Ruby 1.9.3
build. Change the setting to `when_needed` to quiet things down.

Change-Id: I8d8c79e5dc83ea0a5d980d81b8403c14a84dfa63
Reviewed-on: http://gerrit.causes.com/40469
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>