Allow overriding HTTP client type based on environment variable of `RUBY_BUILD_HTTP_CLIENT`

More portable rename after tarball extraction

* make_package() assumes the archive was extracted to $package_name.
* fetch_tarball() does not check the name of the toplevel directory in
  the archive, so rename after if it's not exactly "$package_name".

* This reverts commit d155b41e.
* tar's --strip-components does not seem supported on OpenBSD, even
  though bsdtar does support it.
  See https://github.com/rbenv/ruby-build/commit/d155b41e#commitcomment-30215955

Upgrade to OpenSSL 1.0.2p and 1.1.0i

Add definition for TruffleRuby 1.0.0-rc5

Remove early check for LLVM when installing TruffleRuby

* TruffleRuby already checks for LLVM itself when it needs it and
  looks up more paths to find the LLVM executables clang and opt.
* See https://github.com/oracle/truffleruby/issues/1386.

Add definition for TruffleRuby 1.0.0 RC3

Add mruby 1.4.0 and mruby 1.4.1

Added rbx-3.101 to 105.

Add TruffleRuby to ruby-build

Add definition for Ruby 2.6.0-preview2

https://www.ruby-lang.org/ja/news/2018/05/31/ruby-2-6-0-preview2-released/

Add definition for JRuby 9.2.0.0 and JRuby 9.2.1.0-dev

JRuby 9.2.0.0 has been released.
http://jruby.org/2018/05/24/jruby-9-2-0-0

Allow overriding HTTP client type based on environment variable `RUBY_BUILD_HTTP_CLIENT` (pyenv/pyenv#1126)

Add definition for JRuby 9.1.17.0

JRuby 9.1.17.0 has been released.
http://jruby.org/2018/04/23/jruby-9-1-17-0