Modify .jpmignore so that addon passes auto-signing

[jaredhirsch]

Per [1], a whitelist approach seems more effective for .jpmignore.

[1] https://github.com/mozilla-jetpack/jpm/issues/298

You can test that this passes the auto-signing routine by changing the name in package.json to anything, then attempting to submit on AMO.

@lmorchard @meandavejustice r?

[meandavejustice]

@6a68 This looks like a much better approach, going to test the autosigning this morning before I merge.

[jaredhirsch]

@meandavejustice Bump :-)