Only dump schema after migrating in development environment.

Currently schema is always dumped after migration when the environment isn't a test environment. This is troublesome, because it means it's dumped in production; aside from not being useful, this breaks when the filesystem is read-only(-ish), like on Heroku. Ordinarily this is not obvious, but if you're running some scripts heroku-side you'll note that `rake db:migrate` returns nonzero error code. This patch fixes the issue by only dumping schema in development, arguably the only environment where it makes sense anyway.