Update embedded-hal-mock requirement from 0.7 to 0.8 (#84)

Updates the requirements on [embedded-hal-mock](https://github.com/dbrgn/embedded-hal-mock) to permit the latest version.
- [Release notes](https://github.com/dbrgn/embedded-hal-mock/releases)
- [Changelog](https://github.com/dbrgn/embedded-hal-mock/blob/master/CHANGELOG.md)
- [Commits](https://github.com/dbrgn/embedded-hal-mock/compare/v0.7.0...v0.8.0)

---
updated-dependencies:
- dependency-name: embedded-hal-mock
  dependency-type: direct:production
...

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Cargo.toml

@@ -23,7 +23,7 @@ bit_field = "0.10.1"
 
 [dev-dependencies]
 linux-embedded-hal = "0.3"
-embedded-hal-mock = "0.7"
+embedded-hal-mock = "0.8"
 
 [features]
 default = ["graphics"]