Fix staking (#3284)

* 6 second blocks.

* Version bump

* Add test for slashable_balance()

substrate/node/runtime/src/lib.rs

@@ -79,7 +79,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to equal spec_version. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 125,
+	spec_version: 126,
 	impl_version: 126,
 	apis: RUNTIME_API_VERSIONS,
 };