Support only Ruby 2.1.0 , adding to build list.

This is an application, not a library. Whether in production or
development, installing a specific version of Ruby is relatively
straightforward. Thus, with the start of support for Ruby 2.1.0 , not
only change the default, but also end support for other versions.

If this upsets you, please wave. :)
Upgrade gems.
Tweak documentation, updating copyright.
Fix activesupport missing curses.

Install curses to avoid activesupport

    `require': cannot load such file -- curses (LoadError)

since curses is not present in Ruby 2.1.0 core.

And so it begins.
Tweak CompileScoreWorker , saving once.

Change `LilyPondException` to inherit `StandardError`, simplifying
processing flow to save only once.
Extend CompileScoreWorker#perform to timeout.

Add config `WORKER_COMPILE_TIMEOUT`, defaulting to `::COMPILE_TIMEOUT`,
after which LilyPond compilation will timeout.