Reset associations after loading to allow GC
Avoid following belongs_to associations that have already been dumped

This shaved a ton of SQL CACHE hits + AR object creation on my large
test database.
Use newer Reflection#target= where supported
Use Reflection#reset instead
Use either Base#association or #association_instance_set
Merge pull request #24 from rtomayko/gc-tune

Dump faster with more predictable RES mem growth