Version 0.6.0

Bump the minor version to reflect breaking changes to
`Vcloud::Core::Vm#configure_guest_customization_section`. Make this a
minor bump despite the backwards-incompatible change as we're not yet at
version 1.0 so that API is expected to be unstable[1].

[1]: http://semver.org/
Update CHANGELOG for 0.6.0

Update the CHANGELOG to reflect changes to
`Vcloud::Core::Vm#configure_guest_customization_section` and the minimum
supported Ruby version in version 0.6.0.
Merge pull request #91 from gds-operations/update-changelog

[#74382904] Update CHANGELOG and release version 0.6.0