Replace config field with default value (srml/im-online) (#3388)

* Fix whitespacing

* Replace configurable fields with default values

* Bump version

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix more whitespacing

* Fix style

substrate/srml/im-online/src/lib.rs

@@ -70,7 +70,7 @@
 use primitives::offchain::{OpaqueNetworkState, StorageKind};
 use codec::{Encode, Decode};
 use sr_primitives::{
-	ApplyError, traits::{Extrinsic as ExtrinsicT},
+	ApplyError, traits::Extrinsic as ExtrinsicT,
 	transaction_validity::{TransactionValidity, TransactionLongevity, ValidTransaction},
 };
 use rstd::prelude::*;
@@ -174,7 +174,7 @@ decl_event!(
 decl_storage! {
 	trait Store for Module<T: Trait> as ImOnline {
 		/// The block number when we should gossip.
-		GossipAt get(gossip_at) config(): T::BlockNumber;
+		GossipAt get(gossip_at): T::BlockNumber;
 
 		/// The current set of keys that may issue a heartbeat.
 		Keys get(keys) config(): Vec<AuthorityId>;