Fixes in License format and content

- Rakefile: Use the SPDX format of the license, which is also recommended by
rubygems.org http://spdx.org/licenses/LGPL-2.1+
- Update the COPYING content with the one from https://www.gnu.org/licenses/lgpl-2.1.txt
which has the updated FSF address
@tampakrap no problem at all, I'll update it later to follow the community standard of 2 spaces