Rollback open api version to '0.1.5'

This will avoid a breaking change on the signer side.
A new era will be created then and deployed when enough signers are able to run it.

Author: jpraynaud

openapi.yaml

@@ -4,7 +4,7 @@ info:
   # `mithril-common/src/lib.rs` file. If you plan to update it
   # here to reflect changes in the API, please also update the constant in the
   # Rust file.
-  version: 0.2.0
+  version: 0.1.5
   title: Mithril Aggregator Server
   description: |
     The REST API provided by a Mithril Aggregator Node in a Mithril network.