Small Patch, New Version Coming

[Ken Collins]

All,

I just pushed this commit after having some issue when dropping a
column in a migration. This fixed my issue and does not break any
tests, but I never setup a negative failing test. Can anyone take a
look at let me know what you think?

Implement a new remove_default_constraint method that uses
sp_helpconstraint
http://github.com/rails-sqlserver/2000-2005-adapter/commit/29b26ad9cbc59c0c37c91599ecc2980423f06820

I'll be pushing out 2.2.20 tomorrow if all looks good.

- Thanks,
Ken