- Use ubuntu:xenial as base image
 - Obtain keys from keyserver.ubuntu.com (keys.gnupg.net server times out really often)

RFC 6445, section 4.2.1 says clearly: "An Upgrade header field containing
the value "websocket", treated as an ASCII case-insensitive value".

https://tools.ietf.org/html/rfc6455#section-4.2.1

And now, we have `String#compare(other, case_insensitive: true)`.

Allow OSX failures on Travis CI

Docs for gzip writer used to say `require "zlib"`

Now if LibC.clock_gettime is available it is always used.
Crystal::System::Time works with seconds since `0001-01-01 00:00:00` and with Int64 in all platforms.

Fix #4507

Fix #4509

This reverts commit f833a82c.

Add llvm operand bundle def and catch pad/ret/switch

* Change Random::System to a module

Fixes #4505

* Add System::Random to docs

* Use sysctl to return the number of CPUs on BSD systems

See:
  http://man.openbsd.org/sysctl.3
  https://www.freebsd.org/cgi/man.cgi?query=sysctl&sektion=3

Fixed #4547

Trying to seek from the current position failed because the system
position of a buffered IO is different from the actual position in
the stream.

See https://github.com/crystal-lang/crystal/commit/c4b27d4b369a207050e077f86401cecde053ef0b#commitcomment-22393824.

See https://github.com/crystal-lang/crystal/commit/3961dd0a79d964a132ddea14514db4c79d04bba8#commitcomment-22308027.

Currently #[](key: String) raises `ArgumentError`. I think it should
raise `KeyError` because it looks like `Hash` operation.