Update piston_rs dependency
Summary
This release switches to the new 0.4 version of `piston_rs`. This adds a status property to the `ExecResponse` class, and properly differentiates between an err in the execution response, and an error in the execution result.
**Full Changelog**: https://github.com/Jonxslays/piston_rspy/compare/v0.2.0...v0.3.0