Avoid RS.getObject(index, Object.class) as it breaks MySQL on Java 7

Previously, when running on Java 7+, the JDBC query driven by
DataSourceHealthIndicator resulted in a call to
ResultSet.getObject(index, Object.class). When using MySQL's JDBC
driver this failed with an SQLException with the message "Conversion
not supported for type java.lang.Object". The problem does not occur
on Java 6 as the overload of getObject that takes a type does not
exist; ResultSet.getObject(index) is called instead and MySQL happily
returns whatever type it deems to be appropriate for the column.

This commit updates DataSourceHealthIndicator so that
ResultSet.getObject(index) will always be used, irrespective of the
version of Java that Boot is running on.

Closes #1306