backport truncate until we've upgraded.

truncate was added in upstream in
9a4e183fe9f81365f7815c391512b3d46622d8c8
@tenderlove @Fryguy 

Note:  see the last commit...

    Patch the existing connection object and any future connections.

    The first line of the initializer, `ActiveRecord::Base.connection.respond_to? :truncate`,
    instantiates a connection object so we have to extend that object in addition to
    any future connection objects.

    Before:
    irb(main):001:0> ActiveRecord::Base.connection.respond_to? :truncate
    => false

    After:
    irb(main):001:0> ActiveRecord::Base.connection.respond_to? :truncate
    => true