Removed support 'freezing daemon-kit into vendor/'

This is such a relic of the pre-bundler days. We can now simply have folks update their Gemfile to point to our master branch directly, no need to special 'vendor/daemon-kit' magic.

[ ] - Cleanup boot.rb
[ ] - Make sure it can be upgraded cleanly with `rake daemon_kit:upgrade`
[x] - Document in upgrading.txt
[ ] - Document in post-install, blog post etc

This might just break 0.2.x upgrades