WIP -- define interface via FFI
Remove dep_selector ruby interface

Leave the swig file for reference/documentation, but it is unused now.
Make so loading more platform agnostic
Search lib dir for dep_gecode
Remove abandoned ffi integration
Emit a Makefile by hand on jruby
Add jruby to travis
Remove rake-compiler
Add rake as a dep, it's no longer a transitive dep
Clean before compiling
Add platform independent code flag
Don't try to detect c compiler on JRuby
Restrict which branches get built so we don't double build PRs
ingore jar files

JRuby's DLEXT is "jar", so compilation of the C++ library w/ JRuby
creates the so file as dep_gecode.jar (it's not actually a jar). This
doesn't cause any runtime issues (though it's weird).
Lock ffi to ~> 1.9

This is based on 1.9.3 being the version I tested with. If this causes
conflicts with other projects, we can change this as long as we make
sure it all works.
Revert temporary version number tweak
Normalize shared object suffix using FFI
Merge branch 'ffi'