Rename bundle.json to corral.json

[SeanTAllen]

Currently, the transition path from stable to corral is difficult.
Both use the name bundle.json but they have a different format.
Per issue #17, we have decided to rename corral's dependency file
from bundle.json to corral.json.

Closes #17

[SeanTAllen]

I left the term "Bundle" in place. That seems reasonable.

@cquinn I also didn't change bundle.json to corral.json in package_dependency_management.md as I believe all references there are in relation to stable not corral. Can you verify that it is correct?

[cquinn]

Looks good.

[cquinn]

And, yes, the bundle.json mentions in package_dependency_management.md all refer to stable.