chore(package): update gulp-sourcemaps to version 2.5.1 (#1812)

https://greenkeeper.io/

package.json

@@ -47,7 +47,7 @@
     "gulp-sass": "~3.1.0",
     "gulp-sass-lint": "~1.3.1",
     "gulp-size": "~2.1.0",
-    "gulp-sourcemaps": "~2.5.0",
+    "gulp-sourcemaps": "~2.5.1",
     "gulp-uglify": "~2.1.0",
     "gulp-useref": "~3.1.0",
     "gulp-util": "~3.0.7",