[build] clean out model specs before building them
[build] clean out model specs before building them

Summary:
Sometimes building the models segfaults mysteriously. I've observed in the
latest instance that `make -C infer/models clean` solved the issue so this is
likely a good idea.

Reviewed By: mbouaziz

Differential Revision: D4745924

fbshipit-source-id: cdb50d8

ryoon/infer,blzq/infer,jsachs/infer,blzq/infer,blzq/infer,facebook/infer,ryoon/infer,jvillard/infer,facebook/infer,blzq/infer,ropas/infer,ryoon/infer,facebook/infer,facebook/infer,jvillard/infer,blzq/infer,facebook/infer,jvillard/infer,jsachs/infer,ropas/infer,ryoon/infer,blzq/infer,facebook/infer,jsachs/infer,ropas/infer,ropas/infer,facebook/infer,jsachs/infer,jvillard/infer,jvillard/infer,jsachs/infer,ropas/infer,jsachs/infer,jvillard/infer,ryoon/infer,jvillard/infer,ryoon/infer,ropas/infer