Upgrade NAN to ^2.15.0. Pull request from the main branch: msgpack#57

Author: newbee1905

package.json

@@ -69,7 +69,7 @@
 		"lib": "lib"
 	},
 	"dependencies": {
-		"nan": "^2.14.0"
+		"nan": "^2.15.0"
 	},
 	"devDependencies": {
 		"nodeunit": "https://github.com/godsflaw/nodeunit/archive/master.tar.gz"