fix: Remove tree-kill dependency

The `tree-kill` package, introduced by `snyk-sbt-plugin` has a RCE
vulnerability. This patch raises the version of `snyk-sbt-plugin` to
remove the vulnerability.

See snyk/snyk-sbt-plugin#72 for further
information

package.json

@@ -90,7 +90,7 @@
     "snyk-python-plugin": "^1.14.0",
     "snyk-resolve": "1.0.1",
     "snyk-resolve-deps": "4.4.0",
-    "snyk-sbt-plugin": "2.9.0",
+    "snyk-sbt-plugin": "2.9.1",
     "snyk-tree": "^1.0.0",
     "snyk-try-require": "1.3.1",
     "source-map-support": "^0.5.11",