force US-ASCII encoding for Ruby 2.0

This error occurred on Ruby 2.0.

/vpim-rails/lib/vpim/vcard.rb:678: invalid multibyte escape: /^\xFE\xFF/ (SyntaxError)
invalid multibyte escape: /^\xFF\xFE/

Which uses UTF-8 encoding by default. This patch resolves the issue.

Signed-off-by: Chris Branson <branson.chris@gmail.com>