Use require_relative to require lib/shindo/bin.rb

This way we avoid requiring the file which caused deadlocks with the new protective behaviour of RubyGems require[1]. Requiring the file was holding all the other requires in other threads, so no require could have been used in Shindo tests.

[1]  https://github.com/rubygems/rubygems/commit/c721203e30963daea81f3267ca8492acf2165483.