dep: import go-bitfield into go mod

go.mod

 module github.com/ipfs/go-unixfs
 
 require (
-	github.com/Stebalien/go-bitfield v0.0.0-20180330043415-076a62f9ce6e
+	github.com/Stebalien/go-bitfield v0.0.1
 	github.com/gogo/protobuf v1.2.1
 	github.com/ipfs/go-cid v0.0.1
 	github.com/ipfs/go-ipfs-chunker v0.0.1

go.sum

 github.com/AndreasBriese/bbloom v0.0.0-20180913140656-343706a395b7/go.mod h1:bOvUY6CB00SOBii9/FifXqc0awNKxLFCL/+pkDPuyl8=
 github.com/Kubuxu/go-os-helper v0.0.1/go.mod h1:N8B+I7vPCT80IcP58r50u4+gEEcsZETFUpAzWW2ep1Y=
-github.com/Stebalien/go-bitfield v0.0.0-20180330043415-076a62f9ce6e h1:2Z+EBRrOJsA3psnUPcEWMIH2EIga1xHflQcr/EZslx8=
-github.com/Stebalien/go-bitfield v0.0.0-20180330043415-076a62f9ce6e/go.mod h1:3oM7gXIttpYDAJXpVNnSCiUMYBLIZ6cb1t+Ip982MRo=
+github.com/Stebalien/go-bitfield v0.0.1 h1:X3kbSSPUaJK60wV2hjOPZwmpljr6VGCqdq4cBLhbQBo=
+github.com/Stebalien/go-bitfield v0.0.1/go.mod h1:GNjFpasyUVkHMsfEOk8EFLJ9syQ6SI+XWrX9Wf2XH0s=
 github.com/aead/siphash v1.0.1/go.mod h1:Nywa3cDsYNNK3gaciGTWPwHt0wlpNV15vwmswBAUSII=
 github.com/btcsuite/btcd v0.0.0-20190213025234-306aecffea32 h1:qkOC5Gd33k54tobS36cXdAzJbeHaduLtnLQQwNoIi78=
 github.com/btcsuite/btcd v0.0.0-20190213025234-306aecffea32/go.mod h1:DrZx5ec/dmnfpw9KyYoQyYo7d0KEvTkk/5M/vbZjAr8=