db/storage.go: Add debug Printfs

to try to figure out what goes on in Travis CI when we get this error (note that [`db/storage.go:57`](https://github.com/tsuru/tsuru/blob/master/db/storage.go#L57) is mentioned):

```
----------------------------------------------------------------------
PANIC: actions_test.go:31: ActionsSuite.TearDownSuite
... Panic: runtime error: invalid memory address or nil pointer dereference (PC=0x43CA35)
/home/travis/.gvm/gos/go1.4/src/runtime/asm_amd64.s:401
  in call16
/home/travis/.gvm/gos/go1.4/src/runtime/panic.go:387
  in gopanic
/home/travis/.gvm/gos/go1.4/src/runtime/panic.go:42
  in panicmem
/home/travis/.gvm/gos/go1.4/src/runtime/sigpanic_unix.go:26
  in sigpanic
/home/travis/gopath/src/github.com/tsuru/tsuru/db/storage/storage.go:90
  in Storage.Close
/home/travis/.gvm/gos/go1.4/src/runtime/asm_amd64.s:401
  in call16
/home/travis/.gvm/gos/go1.4/src/runtime/panic.go:387
  in gopanic
/home/travis/.gvm/gos/go1.4/src/runtime/panic.go:42
  in panicmem
/home/travis/.gvm/gos/go1.4/src/runtime/sigpanic_unix.go:26
  in sigpanic
/home/travis/gopath/src/github.com/tsuru/tsuru/db/storage/storage.go:97
  in Storage.Collection
/home/travis/gopath/src/github.com/tsuru/tsuru/db/storage.go:57
  in Storage.Apps
actions_test.go:34
  in ActionsSuite.TearDownSuite
/home/travis/.gvm/gos/go1.4/src/runtime/asm_amd64.s:401
  in call16
```

Cc: @andrewsmedina