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>
gradle: No longer need to tell bnd to delay run dependencies

https://github.com/bndtools/bnd/pull/591 changed bnd to make the default
that run dependencies are delayed.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #16 from bjhargrave/gradle-fixups

Gradle fixups