Reformat CHANGELOG to match CPAN::Changes::Spec

According to our open source guidelines[1], we should use
CPAN::Changes::Spec[2] to format our CHANGELOG.

[1]: https://github.com/gds-operations/open-source-guidelines
[2]: https://metacpan.org/pod/CPAN::Changes::Spec
Fix Modulefile dependency

Apparently because puppetlabs/stdlib version 2.5 doesn't exist, '>= 2.5'
is an invalid dependency; we need to specify '>= 2.5.0' instead. >_<
Bump version to 0.2.1