Add a "Version" field to GameMap Meta (#83)

* initial concept for versioning
* simple uint version implementation
* tidy up dependencies

maps/game_map.go

@@ -1,6 +1,8 @@
 package maps
 
-import "github.com/BattlesnakeOfficial/rules"
+import (
+	"github.com/BattlesnakeOfficial/rules"
+)
 
 type GameMap interface {
 	// Return a unique identifier for this map.
@@ -20,6 +22,9 @@ type Metadata struct {
 	Name        string
 	Author      string
 	Description string
+	// Version is the current version of the game map.
+	// Each time a map is changed, the version number should be incremented by 1.
+	Version uint
 }
 
 // Editor is used by GameMap implementations to modify the board state.