Release v0.5.1

v0.5.1 (March 27, 2021)

• ENH: Add nocache option to DockerImage.build() (#278)
• ENH: Add option to remove version pinning from pip requirements file (#277)
• FIX: Use default pickle protocol even for cloudpickle (#273)
• ENH: Add --ignore-installed option to ck.DockerImage (#275)
• ENH: Remove six (#274)