box: Fix upload of > 2GB files on 32 bit platforms

Before this change the Part structure had an int for the Offset and
uploading large files would produce this error

    json: cannot unmarshal number 2147483648 into Go struct field Part.offset of type int

Changing the field to an int64 fixes the problem.

backend/box/api/types.go

@@ -172,8 +172,8 @@ type UploadSessionResponse struct {
 // Part defines the return from upload part call which are passed to commit upload also
 type Part struct {
 	PartID string `json:"part_id"`
-	Offset int    `json:"offset"`
-	Size   int    `json:"size"`
+	Offset int64  `json:"offset"`
+	Size   int64  `json:"size"`
 	Sha1   string `json:"sha1"`
 }