Clarify use of already downloaded .insecure_key

The `start-cluster` target automatically downloads the
`phusion/baseimage` insecure key to extract cluster credentials. This
changeset updates the documentation so that users are aware that the key
already exists locally and doesn't need to be re-downloaded.

Attempts to resolve #10.
Update versions

- Bump Riak version to `1.4.10`
- Bump `phusion/baseimage` version to `0.9.15`
- Bump Riak CS version to `1.5.2`
- Bump Stanchion version to `1.5.0`

Attempts to resolve #11.
Merge branch 'feature/clarify-key-docs' into develop
0.6.0
Merge branch 'release/0.6.0' into develop