update rake version

"rake spec" showed following warning
/home/muratayusuke/.rvm/gems/ruby-1.9.3-p392/gems/rake-0.8.7/lib/rake/alt_system.rb:32: Use RbConfig instead of obsolete and deprecated Config.
(in /mnt/projects/homesick)
/home/muratayusuke/.rvm/rubies/ruby-1.9.3-p392/bin/ruby -S rspec spec/homesick_spec.rb
so update rake version to fix it.
update changelog
Regenerate gemspec for version 0.8.1