0.23.1

This is mainly a bugfix release.

The biggest changes are not in the compiler's codebase, but in the release process. Specifically, compatibility with older platforms lost in the 0.23.0 release has been recovered (Debian 7/Ubuntu 14.04, Centos 6), while keeping the builds with LLVM 3.8.

• Fixed macro lookup from included modules (thanks @straight-shoota, @asterite, see #4639)
• Fixed WebSocket Upgrade header is now case-insensitively compared (thanks @makenowjust, see #4617)
• Upgraded PCRE version with security fixes (thanks @Sija, @ivy, see crystal-lang/omnibus-crystal#20)
• Added PCG32 random generator, now the default one (thanks @konovod, see #4536)
• Added crystal tool format in man page (thanks @makenowjust, see #4643)
• Docs & platform improvements