Bump cross-env from 5.2.1 to 6.0.3 (#156)

Bumps [cross-env](https://github.com/kentcdodds/cross-env) from 5.2.1 to 6.0.3.
- [Release notes](https://github.com/kentcdodds/cross-env/releases)
- [Changelog](https://github.com/kentcdodds/cross-env/blob/master/CHANGELOG.md)
- [Commits](kentcdodds/cross-env@v5.2.1...v6.0.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

package.json

@@ -15,7 +15,7 @@
     "test": "cross-env DEBUG=mon* mocha"
   },
   "devDependencies": {
-    "cross-env": "^5.2.0",
+    "cross-env": "^6.0.3",
     "mocha": "^5.2.0",
     "which": "^1.2.0"
   },