Delayed require pry

Under spring booted environment, `rails console` starts pry console,
but console prompt doesn't show up until after hiting return

This issue occurs because

    require 'pry'

requires readline too early.

Solve the issue by delaying to require pry into Railtie console callback.

closes #50
refs rails/spring#244
Merge pull request #51 from yasuoza/delaying_require_pry

Delay requiring pry