gradle: Use fixed file name for downloaded bnd jar

The URI provided by the bnd_jar property may not have a valid file name
as the last component of the URI's path. This is true for JPM URLs.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #879 from bjhargrave/bndjarfile

gradle: Use fixed file name for downloaded bnd jar