Corected a Bolt Arguments Issue in api/db.go

Had an Issue installing ipxed:
/home/setkeh/gocode/src/github.com/kelseyhightower/ipxed/# go install
# github.com/kelseyhightower/ipxed/api
api/db.go:19: not enough arguments in call to bolt.Open

Added "nil" to the Arguments on bolt.open on line 19 of api/db.go and successfully built afterwards.

Hope this helps thanks for the great work :)