Fix execution of migrations with stored procedures

See this issue for more background:
https://github.com/weavejester/ragtime/issues/17

## Changes
Use jdbc to directly execute statement string from migration file, as opposed to splitting on semi-colons (which breaks in the context of complex statements which temporarily modify the delimiter)

## Testing
Was going to add a test for such a statement, but couldn't find a stored-procedure like equivalent with the h2 db engine that's being used for testing.  Ran all the existing tests and now things are working properly when executing migrations with such statements