Update fs-extra to version 7.0.0 (postcss#231)

Closes postcss#213

Author: RyanZim

package.json

@@ -21,7 +21,7 @@
     "chalk": "^2.1.0",
     "chokidar": "^2.0.0",
     "dependency-graph": "^0.7.0",
-    "fs-extra": "^5.0.0",
+    "fs-extra": "^7.0.0",
     "get-stdin": "^6.0.0",
     "globby": "^8.0.0",
     "postcss": "^7.0.0",