Use scrub_rb instead of ensure_valid_encoding

Ruby-Marc API should be identical to users, all tests still pass.
Using scrub_rb, which will use standard ruby String#scrub in 2.1,
instead of the older pre-2.1 ensure_valid_encoding gem.
Merge branch 'master' of https://github.com/ruby-marc/ruby-marc

Conflicts:
	lib/marc/reader.rb