Oniguruma: Project moved to GitHub

[elad]

Should help fix occurrences like jqlang/jq#936.

[kmaehashi]

👍
We are using formulas that depends on oniguruma. Hoping that this PR gets merged soon.

[DomT4]

==> Downloading https://github.com/kkos/oniguruma/archive/v5.9.6.tar.gz
Already downloaded: /Library/Caches/Homebrew/oniguruma-5.9.6.tar.gz
==> Verifying oniguruma-5.9.6.tar.gz checksum
tar xf /Library/Caches/Homebrew/oniguruma-5.9.6.tar.gz
==> ./configure --disable-dependency-tracking --prefix=/usr/local/Cellar/oniguruma/5.9.6
Failed to execute: ./configure

If the archive tarballs are automatically generated, you'll probably need the autotools here.