Use geos-config to compile/link correctly against GEOS.

This request changes extconf.rb to rely on geos-config to discover correct GEOS header/library paths *and* names, rather than just a list of paths.

This should fix:
https://github.com/rgeo/rgeo/issues/60
https://github.com/rgeo/rgeo/issues/73
and should re-instate the CAPI tests on Travis which are currently being skipped.