Release v0.6.0

Fixed

• Resolved "heartbeat" alerts are handled correctly, i.e. not at all ([#24])
• The go module dependency definition doesn't use replace to pull in our fork of go-icinga2-client anymore. ([#21])
• go get is now able to fetch and install Signalilo ([#19])
• The signalilo:latest docker image is built a more useful version tag in the binary