Disable dependabot

Due to a bug in dependabot (see issue 2198 in the
`dependabot/dependabot-core` repository), Dependabot would open pull
requests on every fork, which would be inconvenient.

So we're disabling the updates until this issue is fixed; since we
currently have almost 50 open pull requests, it shouldn't affect us that
much.

.github/dependabot.yml

@@ -1,9 +0,0 @@
-version: 2
-updates:
-- package-ecosystem: bundler
-  directory: "/"
-  schedule:
-    interval: daily
-    time: "06:00"
-    timezone: Europe/Madrid
-  open-pull-requests-limit: 25