Bump cocur/slugify from 3.0.1 to 3.1

Bumps [cocur/slugify](https://github.com/cocur/slugify) from 3.0.1 to 3.1.
- [Release notes](https://github.com/cocur/slugify/releases)
- [Commits](cocur/slugify@v3.0.1...v3.1)

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

composer.json

@@ -15,7 +15,7 @@
   ],
   "require": {
     "php": "^5.6|^7.1",
-    "cocur/slugify": "3.0.1",
+    "cocur/slugify": "3.1",
     "dflydev/dot-access-data": "1.1.0",
     "erusev/parsedown-extra": "0.7.1",
     "matthiasmullie/minify": "1.3.56",