fix(package): update deepmerge to version 4.2.0

Closes #130

Author: greenkeeper[bot]

Diff for: package.json

@@ -77,7 +77,7 @@
     "clean-css": "^4.1.9",
     "debug": "^2.6.9",
     "dedupe": "^2.1.0",
-    "deepmerge": "^2.0.0",
+    "deepmerge": "^4.2.0",
     "html-minifier": "^3.5.5",
     "lru-cache": "^4.1.1",
     "no-case": "^2.3.2",