Bump version to 0.2.1v0.2.1

This change bumps the version of the crate to 0.2.1. The following
notable changes have been made since 0.2.0:
- Added the pws command for accessing the password safe
- Added the lock command for locking the Nitrokey device
- Adjust release build compile options to optimize binary for size
- Bumped nitrokey dependency to 0.2.3
  - Bumped rand dependency to 0.6.1
  - Added rustc_version version 0.2.3, semver version 0.9.0, and
    semver-parser version 0.7.0 as indirect dependencies
- Bumped cc dependency to 1.0.28

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