Publish a new release to fix security issue #241

Our (Gitlab) based security scanner flagged up an issue in Execa 1.0.0, which was in our dependency chain via shx and shelljs. Shelljs was fixed in April (shelljs/shelljs#1216) and it was updated in shx in May (0f1ea07), fixing the security issue, but the latest release of shx (0.4.0) was done in March.

Please publish a new version so that any users of shx can update. It's not a huge security issue for us, it's just the security dashboard being a bit panicky.