Bug 1379482 - Update geckodriver repository address; r=jgraham

The canonical geckodriver source code no longer lives on GitHub, but
in mozilla-central's testing/geckodriver directory.  Linking through
searchfox.org because it provides a better UI than hg.mozilla.org.

MozReview-Commit-ID: LmnviuC7Z9Y

--HG--
extra : rebase_source : 23f0e669bd719c595300d4e7ac38bc12f4937f57

testing/geckodriver/Cargo.toml

@@ -7,7 +7,7 @@ authors = [
 ]
 description = "Proxy for using WebDriver clients to interact with Gecko-based browsers."
 keywords = ["webdriver", "w3c", "httpd", "mozilla", "firefox"]
-repository = "https://github.com/mozilla/geckodriver"
+repository = "https://hg.mozilla.org/mozilla-central/file/tip/testing/geckodriver"
 readme = "README.md"
 license = "MPL-2.0"