Make deploygate API secrets optional, add task only if present, fixes #1
Replace Crashlytics SDK with DeployGate SDK

Using the deploygate SDK for beta releases anyway so it's better to use the features at DG instead of using two different servies.
Also: Less dependencies, easier build config, yeah!
See https://deploygate.com/docs/sdk
Bump version to 0.4.1 for release
Merge branch 'release/0.4.1' into develop