Refactor to work with stable rust
## v1.4.2 (2019-05-21) #### Bug Fixes * **tests:** fix locked error in some multi-threading cases ([b40cc5ac](b40cc5ac))
## v1.4.2 (2019-05-21) #### Bug Fixes * **tests:** fix locked error in some multi-threading cases ([b40cc5ac](b40cc5ac))