Fix problem with encoding in WordArray

I started getting exceptions in `WordArray` when inspecting `spec/rubocop/cop/style/encoding_spec.rb` and running under ruby-1.9.3-p547. There are no such problems on Travis for the exact same ruby version, so it must be related to other environment factors, but I have been unable to determine what it is exactly. Anyway, I think this change takes care of the problem.

Force the encoding of the regular expression taken from a configuration file to be the same as for the string from the inspected code. This is to avoid getting the error "incompatible encoding regexp match (ASCII-8BIT regexp with UTF-8 string)".