fix: correct semantic-release version

Misread their commit log.

Author: gajus

package.json

@@ -21,7 +21,7 @@
     "gitdown": "^2.4.0",
     "globby": "^4.0.0",
     "mocha": "^2.5.3",
-    "semantic-release": "^9.0.0"
+    "semantic-release": "^6.3.6"
   },
   "engines": {
     "node": ">=4"