Simplify the requires, to make rbenv via homebrew happy

rbenv installed via homebrew uses a symlink in the path, so I was seeing a
conflict between require & require relative, causing a warning:

[1] pry(main)> require 'magritte'
/usr/local/var/lib/rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/magritte-0.5.6/lib/magritte/iostreams.rb:7: warning: already initialized constant Magritte::BlockStream::READ_BLOCK_SIZE
/usr/local/opt/rbenv/versions/2.0.0-p195/lib/ruby/gems/2.0.0/gems/magritte-0.5.6/lib/magritte/iostreams.rb:7: warning: previous definition of READ_BLOCK_SIZE was here
=> true
seems using `git ls-files` might not be the best thing
Merge pull request #1 from mydrive/simplified-requires

Simplified requires