Work around MySQL 5.6+ bug when combining DROP FOREIGN KEY and DROP INDEX in same ALTER TABLE statement

I'm not sure exactly when this bug started, but at least on 5.6.17,
issuing a DROP FOREIGN KEY and a DROP INDEX in the same ALTER TABLE
statement, when DROP INDEX is dropping the unique index that the
foreign key depends on, now raises an error.  I'm assuming this
started in 5.6, so on 5.6+, always issue the DROP INDEX in a
separate ALTER TABLE statement.
Support :fractional_seconds Database option on MySQL 5.6.5+ to support fractional seconds by default (Fixes #797)

MySQL 5.6.4+ supports fractional seconds in timestamps.  MySQL
5.6.5+ supports datetime columns with CURRENT_TIMESTAMP default
values.  As I doubt many people are actually using MySQL 5.6.4,
I've chosen to just enable this on 5.6.5+.

Note that just enabling fractional seconds breaks backwards
compatibility.  If you try to put a fractional second timestamp
into a column that doesn't support fractional seconds, MySQL
raises an error.  For this reason, you must set the
:fractional_seconds Database option to enable this support.