strip off (precision,scale) when comparing sql base types to avoid erroneous "schema changes"

Hi David - here's another simple change that prevents unnecessary "detected schema changes" on subsequent runs.  The issue is when you specify `:type => :decimal` without a `precision` or `scale`, this will result in the creation of something like `decimal(10,2)`.  On subsequent runs, this will be seen as a change and the database will be modified even though it is unnecessary.  This can cause performance issues on very large tables in production.  Thanks - cheers.