Update to 2.000018

Upstream changes:
2.000018 - 2015-10-29
- repair CPAN.pm bootstrapping broken by extraneous prerequisite.
- ensure directories are created in correct order to avoid errors

2.000017 - 2015-10-06
- fix shell quoting for fish shell
- work around issue where some cpanm versions wouldn't upgrade
  ExtUtils::MakeMaker in some cases it should have
- added --quiet option to surpress directory creation message
- build_environment_vars_for, build_activate_environment_vars_for, and
  environment_vars_string_for options will always include directories being
  added, even if they were already in current ENV vars
- fixes for author tests
- create directory structure under created local::libs, ensuring arch dirs
  will always be added to @INC by perl
- only cause CPAN.pm to prefer ExtUtils::MakeMaker over Module::Build during
  bootstrap process, not after
- bootstrap process refactored

(wen)