2.1.0

This release contains a few small improvements and updates to match
changes in bodhi server code.

Added:

• added eol field to the Release struct
• added critpath_gtoups field to the Update struct

Changed:

• dropped the oldtime feature of chrono to remove the dependency on the
  obsolete version 0.1 of the time crate
• adapted tests for new and out-of-date Fedora releases
• mark structs with the #[non_exhaustive] attribute instead of using
  private fields
• ported from lazy_static to once_cell

Fixed:

• fixed the deprecated(since) annotation for the composes field of the
  Release struct