chore: release 3.15.1 (#519)

🤖 I have created a release \*beep\* \*boop\*
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### [3.15.1](https://www.github.com/googleapis/nodejs-asset/compare/v3.15.0...v3.15.1) (2021-06-30)


### Bug Fixes

* **deps:** google-gax v2.17.0 with mTLS ([#514](https://www.github.com/googleapis/nodejs-asset/issues/514)) ([85e0ebd](https://www.github.com/googleapis/nodejs-asset/commit/85e0ebd92f326ce52f3a920a8cea9d392ff821b5))
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This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please).

Author: release-please[bot]

asset/snippets/package.json

@@ -15,7 +15,7 @@
     "test": "mocha --timeout 180000"
   },
   "dependencies": {
-    "@google-cloud/asset": "^3.15.0",
+    "@google-cloud/asset": "^3.15.1",
     "@google-cloud/bigquery": "^5.5.0",
     "@google-cloud/compute": "^2.0.0",
     "@google-cloud/storage": "^5.0.0",