Update mocha and rake dependencies.

[faisal]

• Bump Mocha and Rake dependencies to latest versions.
• Bump MDL dependency by removing the ''~> 0.15.0' lock, since the way we use it should not be so sensitive to minor version drift.

Check list:

• Add an entry to the changelog
• Squash all commits into a single one
• Describe your PR, link issues, etc.