Fixes symlinks omission from diff.
use method_defined? instead of hardcoded version string to support both ruby 1.9 and 2.0.
explicitly set ASCII_8BIT encoding in PACK_IDX_SIGNATURE.

otherwise "..."'s encoding is UTF-8 in ruby 2.0.
Merge pull request #9 from fdiary/ruby-2.0

support ruby 2.0
Merge pull request #7 from kouno/fix-symlinks

Fixes symlinks omission from diff.
Add ruby 2.0.0 to travis