Eliminate 1.8 compatibility.

Welcome to the future.
Fix double typo in LZMAStream#initialize. Fixes #6.

This was a curious combination of two typos; one missing NOT followed
by an invalid field name (missing the "_in"). Due to the missing NOT,
the missing field was never to be encountered, and the `super' call
was executed instead, which errored on JRuby due to incorrect arguments.
The missing NOT caused the underlying `lzma_stream' struct to be
created uninitialized, which just worked *by chance* (as it always is
if you use variables just created on unused memory in C). Now it properly
gets set up as liblzma would do it itself.
Fix some gemspec stuff.
Update README
Add a COPYING file.

Previously the license was only mentioned in the README, so I thought
adding the obligatory COPYING file should be done.
Version bump