Version 2.1 should be 3.0

[ndac-todoroki]

Description of issue

• What are the expected results?
  incompatible changes should bump Major version up
• What version of Distillery?
  2.1.0

According to the Semantic Versioning 2.0 docs, backward incompatible changes should bump major versions up.

Given that Mix.Release changing to Distillery.Releases without any aliases, this should be a major version-up.

Some libraries which specify {:distillery, "~> 2.0"} like mix_erllambda are already affected by this.

[jhaagmans]

I second this. Also, I really like that there is a documented upgrade path to 2.x but I was unable to find documentation about these breaking changes in 2.1.

[shonfeder]

IMO, it is well worth cutting a new release just to get the versioning correct. We lost a half day trying to puzzle out what broke, because we didn't suspect a minor version change would break the entire release process.

[lpil]

This has been very confusing on our application. Sticking to semver would have been appreciated