Do not force to require everything

If the user wants to use line_change directly from Ruby without Rake,
loading tasks is just a (small) overhead. So make it possible to load
only classes without tasks.

    # Only loads LineChange classes
    require 'line_change'

    # Load rake tasks as well as all LineChange classes
    # (internally calls `require 'line_change'`)
    require 'line_change/tasks'
Add travis config