chore(package): upgrade to semantic-release@15

Author: SimenB

package.json

@@ -38,7 +38,7 @@
     "lint-staged": "^7.0.0",
     "prettier": "^1.10.2",
     "prettylint": "^1.0.0",
-    "semantic-release": "^12.2.2",
+    "semantic-release": "^15.0.2",
     "travis-deploy-once": "^4.3.1"
   },
   "prettier": {