bower.json: switch from SteveSanderson/knockout to bowerjs/knockout

SteveSanderson/knockout (knockout) doesn't contain a compiled version
of knockout, so it's impossible to use with bower if you're not also
using grunt. bowerjs/knockout (knockout.js) has been updated
regularly, and allows for a broader range of build setups.

Signed-off-by: Evan Owen <kainosnoema@gmail.com>
Merge pull request #99 from kainosnoema/bower-ko-compiled

bower.json: switch from SteveSanderson/knockout to bowerjs/knockout
https://github.com/kmalakoff/knockback/issues/98