use caret requirements for dependencies & prepare 0.9.0 release

[lauralt]

Summary of the PR

Updated the deps to use caret requirements. After merging this, we can have a new release of vm-memory. Fixes #199

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [ ] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [ ] All added/changed functionality has a corresponding unit/integration test.
• [ ] Any newly added unsafe code is properly documented.

[wllenyj]

Thanks for your work. I was thinking of waiting for a new version of vmm-sys-util to be released before updating vm-memory. But it doesn't seem to be a problem now. vmm-sys-util and vm-memory have the same dependency libc, and their version can be covered.