Be specific about the BitVec generic arguments (#830)

* Be specific about the `BitVec` generic arguments

Currently we use the default generic arguments for `BitVec`. This means
we use `BigEndian` and `u8`. These default values are not stable, with
`0.17` of the `BitVec` crate this changes. To make sure we don't break
anything in the future, make sure we explictly set the generics.

* Update `spec_version`

polkadot/runtime/common/src/registrar.rs

@@ -862,7 +862,7 @@ mod tests {
 				.collect(),
 			validator_indices: roster.iter()
 				.map(|i| i == &Chain::Parachain(id))
-				.collect::<BitVec>(),
+				.collect::<BitVec::<_, _>>(),
 		}
 	}