Bump gulp-sourcemaps@^2.6.4

Author: xzyfer

package.json

@@ -39,7 +39,7 @@
     "globule": "^1.0.0",
     "gulp": "^3.8.11",
     "gulp-postcss": "^5.1.10",
-    "gulp-sourcemaps": "^1.5.2",
+    "gulp-sourcemaps": "^2.6.4",
     "gulp-tap": "^0.1.3",
     "mocha": "^5.0.4",
     "rimraf": "^2.4.3",