Fixed root file location

This one fixes gem requiring. Root file should be located in `lib` in order to make `require "russian_post"` work.

Before:

    irb(main):001:0> require "russian_post"
    LoadError: cannot load such file -- russian_post
	from /Users/artem/.rbenv/versions/custom-2.0.0-p0/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
	from /Users/artem/.rbenv/versions/custom-2.0.0-p0/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:45:in `require'
	from (irb):1
	from /Users/artem/.rbenv/versions/custom-2.0.0-p0/bin/irb:12:in `<main>'
    irb(main):002:0> require "russian_post/russian_post"
    => true

After:

    irb(main):001:0> require "russian_post"
    => true