Clean up strtree::Interval interface

Set const-correctness, drop useless copy-ctor-like method

git-svn-id: http://svn.osgeo.org/geos/trunk@3986 5242fede-7e19-0410-aef8-94bd7d2200fb
Update ruby binding build scripts

Build succeeds with ruby1.9.1-dev.
Swig wrapper updated.
See https://github.com/libgeos/libgeos/pull/22

git-svn-id: http://svn.osgeo.org/geos/trunk@3987 5242fede-7e19-0410-aef8-94bd7d2200fb