Crystal introduce gzip in stdlib

[waghanza]

Some breaking changes has been introduced in v0.21.0 :

• a gzip module is now available @see https://crystal-lang.org/api/0.21.0/Gzip.html
• MemoryIO does not exists anymore @see https://github.com/crystal-lang/crystal/pull/3563

@schovi With the #6, ci badge is displayed on readme

not a spec master but I have a strange behavior images/sidekiq.png file is 47880 bytes on my system when gzipp'ing it and 47883 with crystal