Bump badger to 2.0.0.rc2

Broken currently here:

    go: finding github.com/zippoxer/bow latest
    go: github.com/dgraph-io/badger@v2.0.0-rc.2+incompatible: go.mod has post-v2 module path "github.com/dgraph-io/badger/v2" at revision v2.0.0-rc.2
    go: error loading module requirements

Which seems to be related to dgraph-io/badger#886

There's also a recent upstream API change in dgraph-io/badger#874
that requires additional changes.

db.go

@@ -167,7 +167,7 @@ type DB struct {
 // Make sure to call Close after you're done.
 func Open(dir string, options ...Option) (*DB, error) {
 	db := &DB{
-		badgerOptions: badger.DefaultOptions,
+		badgerOptions: badger.DefaultOptions(dir),
 		codec:         jsoncodec.Codec{},
 	}
 

go.mod

@@ -5,7 +5,7 @@ go 1.12
 require (
 	github.com/AndreasBriese/bbloom v0.0.0-20190306092124-e2d15f34fcf9 // indirect
 	github.com/deckarep/golang-set v1.7.1 // indirect
-	github.com/dgraph-io/badger v2.0.0-rc.2+incompatible
+	github.com/dgraph-io/badger v2.0.0-rc2+incompatible
 	github.com/dgryski/go-farm v0.0.0-20190323231341-8198c7b169ec // indirect
 	github.com/dustin/go-humanize v1.0.0 // indirect
 	github.com/golang/protobuf v1.3.1 // indirect

go.sum

@@ -6,6 +6,8 @@ github.com/deckarep/golang-set v1.7.1 h1:SCQV0S6gTtp6itiFrTqI+pfmJ4LN85S1YzhDf9r
 github.com/deckarep/golang-set v1.7.1/go.mod h1:93vsz/8Wt4joVM7c2AVqh+YRMiUSc14yDtF28KmMOgQ=
 github.com/dgraph-io/badger v2.0.0-rc.2+incompatible h1:7KPp6xv5+wymkVUbkAnZZXvmDrJlf09m/7u1HG5lAYA=
 github.com/dgraph-io/badger v2.0.0-rc.2+incompatible/go.mod h1:VZxzAIRPHRVNRKRo6AXrX9BJegn6il06VMTZVJYCIjQ=
+github.com/dgraph-io/badger v2.0.0-rc2+incompatible h1:6fXfqViMStaKb73bxivvD9hgwYdWyiEKzwWNVkXhslE=
+github.com/dgraph-io/badger v2.0.0-rc2+incompatible/go.mod h1:VZxzAIRPHRVNRKRo6AXrX9BJegn6il06VMTZVJYCIjQ=
 github.com/dgryski/go-farm v0.0.0-20190323231341-8198c7b169ec h1:sElGDs3V8VdCxH5tWi0ycWJzteOPLJ3HtItSSKI95PY=
 github.com/dgryski/go-farm v0.0.0-20190323231341-8198c7b169ec/go.mod h1:SqUrOPUnsFjfmXRMNPybcSiG0BgUW2AuFH8PAnS2iTw=
 github.com/dustin/go-humanize v1.0.0 h1:VSnTsYCnlFHaM2/igO1h6X3HA71jcobQuxemgkq4zYo=