Automatically use Redshift 4.0 or 4.1 JDBC driver, depending on which is installed

[JoshRosen]

Currently, the default value of jdbcdriver is com.amazon.redshift.jdbc4.Driver, but this will not work if a user has installed the JDBC 4.1 driver. Instead of hardcoding either the 4.0 or 4.1 driver, we should handle the absence of a user-provided jdbcdriver config by first trying to load the JDBC4 driver and falling back to attempt the 4.1 driver (or vice versa).

See #56 (comment) for additional context.