Fix Dir.entries on ruby-2.1.0-preview2

Ruby 2.1.0's `Dir.entries` method now allows an "encoding" option.  Without this change, an `ArgumentError` is raised.

See also http://ruby-doc.org/core-2.1.0/Dir.html

I found this while [working on Ruby 2.1.0 compatibility for Maid](https://github.com/benjaminoakes/maid/issues/114).  As a part of debugging Maid's specs, I extracted a minimal spec that demonstrates the problem.  It's currently in `spec/fakefs/fakefs_bug_ruby_2.1.0-preview2.rb`.

Thanks for maintaining FakeFS.  It makes Maid's specs much easier to write!

Ben