Add magic comment for files containing binary string literals.

It's more likely we'll be adding more binary strings to these files so
the magic comment will ensure these don't get converted to UTF-8 in ruby
2.0.

More #589
Use magic comment for binary string literals instead of force_encoding.

force_encoding("ASCII-8BIT") only fixes one string at a time.
Future binary string literals added to these files would need to be forced
to binary.  The #encoding magic comment will fix future strings added to
these files.

More #589
Merge pull request #804 from jrafanie/us_ascii_magic_comment

Use `#encoding: US-ASCII` magic comment for binary string literals with ruby 2.0