[Security] Bump league/commonmark from 0.18.1 to 0.18.3

Bumps [league/commonmark](https://github.com/thephpleague/commonmark) from 0.18.1 to 0.18.3. **This update includes security fixes.**
- [Release notes](https://github.com/thephpleague/commonmark/releases)
- [Changelog](https://github.com/thephpleague/commonmark/blob/master/CHANGELOG.md)
- [Commits](https://github.com/thephpleague/commonmark/compare/0.18.1...0.18.3)

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

composer.lock

@@ -2485,16 +2485,16 @@
         },
         {
             "name": "league/commonmark",
-            "version": "0.18.1",
+            "version": "0.18.3",
             "source": {
                 "type": "git",
                 "url": "https://github.com/thephpleague/commonmark.git",
-                "reference": "e5029f74ba39e043ce4b3ca6c05dc719d8aafd94"
+                "reference": "b1ec41ce15c3bd6f7cbe86a645b3efc78d927446"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/thephpleague/commonmark/zipball/e5029f74ba39e043ce4b3ca6c05dc719d8aafd94",
-                "reference": "e5029f74ba39e043ce4b3ca6c05dc719d8aafd94",
+                "url": "https://api.github.com/repos/thephpleague/commonmark/zipball/b1ec41ce15c3bd6f7cbe86a645b3efc78d927446",
+                "reference": "b1ec41ce15c3bd6f7cbe86a645b3efc78d927446",
                 "shasum": ""
             },
             "require": {
@@ -2510,7 +2510,7 @@
                 "erusev/parsedown": "~1.0",
                 "michelf/php-markdown": "~1.4",
                 "mikehaertl/php-shellcommand": "^1.2",
-                "phpunit/phpunit": "^5.7|^6.5",
+                "phpunit/phpunit": "^5.7.27|^6.5.14",
                 "scrutinizer/ocular": "^1.1",
                 "symfony/finder": "^3.0|^4.0"
             },
@@ -2550,7 +2550,7 @@
                 "markdown",
                 "parser"
             ],
-            "time": "2018-12-30T01:55:29+00:00"
+            "time": "2019-03-21T22:47:25+00:00"
         },
         {
             "name": "league/flysystem",