Install ExtUtils::MakeMaker with CPANM

This updates ExtUtils::MakeMaker to the latest version.

Fixes https://github.com/travis-ci/travis-ci/issues/2740
Add Perl runtime that has -Duseithreads

This will coexist with -Duseshrplib, and will be named -extras.

For backward compatibility, `5.*-shrplib` aliases are also created.

This fixes https://github.com/travis-ci/travis-ci/issues/2709
Update changelog
Update `nvm` to v0.15.0.

Fixes `nvm copy-packages`; adds `$NVM_SYMLINK_CURRENT` env var to
prevent creation of a "current" symlink; makes `sha1sum` have a higher
priority than `shasum`.
Merge pull request #372 from ljharb/update_to_nvm_0_15_0

Update `nvm` to v0.15.0
Invoke perlbrew-installer with bash

The shebang line is incorrectly set to invoke sh.

See https://github.com/gugod/App-perlbrew/issues/407
Merge branch 'master' of github.com:travis-ci/travis-cookbooks
Merge branch 'ha-feature-perl-extra'