Remove old github.com references, and update to gopkg.in ones

Author: jseriff

float64.go

@@ -6,7 +6,7 @@ import (
 	"encoding/json"
 	"strconv"
 
-	"github.com/nullbio/null/convert"
+	"gopkg.in/nullbio/null.v6/convert"
 )
 
 // Float64 is a nullable float64.

int64.go

@@ -6,7 +6,7 @@ import (
 	"encoding/json"
 	"strconv"
 
-	"github.com/nullbio/null/convert"
+	"gopkg.in/nullbio/null.v6/convert"
 )
 
 // Int64 is an nullable int64.

string.go

@@ -5,7 +5,7 @@ import (
 	"database/sql/driver"
 	"encoding/json"
 
-	"github.com/nullbio/null/convert"
+	"gopkg.in/nullbio/null.v6/convert"
 )
 
 // String is a nullable string. It supports SQL and JSON serialization.