Automate dependency updates for gemnasium-maven-plugin

To make it easier to maintain this plugin we should automate dependency updates of https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-maven-plugin.
We can use https://renovatebot.com for this. It's open source and easy to set up.

The bot will create a new MR every time a dependency update is available.
This makes it very easy to keep java projects up to date.

See haynes/libsass-maven-plugin!20 (merged) for an example how an MR created by the bot looks like.