Bumps version to 0.6.0, updates CHANGES.md.

Version bump to 0.5.0, update to CHANGES.md. (amazon-ion#110)

Release v0.4.1

Bumps version to 0.4.0 and updates CHANGES.md. (amazon-ion#81)

Bumps version to 0.3.1.

Bumps version to 0.2.0. (amazon-ion#51)