port to gecode >=4.0

Hi!

I sent a pull request sometime ago to algorist https://github.com/algorist/dep_selector/pull/5 to port dep-selector to gecode >= 0.4. I was asked to ping you. So here I am.

In 2013, the version 4.0 of the gecode library went out with major modifications breaking dep_selector: the notable changes are:

 -  Restart class is now called RBS. This template class takes an additional parameter representing the engine to use. The constructor doesn't have a default value for the search option.
 -    the statistics do not provide info about memory by default anymore
 -   branching value selections are now functions.

You can see what changes were applied in Debian for the version 0.0.8 at
http://patch-tracker.debian.org/package/ruby-dep-selector/0.0.8-2. I adapted the patch to 0.0.9/0.10 in this pull request.

Cédric