update_app() no-ops if Version is passed

[mbabineau]

(Reported by @xtfxme: #13 (comment))

If app.version is set in the app passed to client.update_app(), Marathon won't apply the changes.

Workaround is to set the version to None:

app = c.get_app(ident)
app.instances = 10
app.version = None
c.update_app(app.id, app)

[mbabineau]

Fixed by 2daa510