[Minor] Move JdbcRDDSuite to the correct package

It was in the wrong package

Author: Andrew Or <andrewor14@gmail.com>

Closes #839 from andrewor14/jdbc-suite and squashes the following commits:

f948c5a [Andrew Or] cache -> cache()
b215279 [Andrew Or] Move JdbcRDDSuite to the correct package

(cherry picked from commit 7c79ef7d43de258ad9a5de15c590132bd78ce8dd)
Signed-off-by: Reynold Xin <rxin@apache.org>