Update `nvm` from `0.25.4` to `0.27.0`.
Update `nvm` from `0.25.4` to `0.27.0`.

See https://github.com/creationix/nvm/releases/tag/v0.27.0 for release notes: tl;dr: much better node 4+ support, plus bug fixes.

ljharb/travis-cookbooks,spurti-chopra/travis-cookbooks,travis-ci/travis-cookbooks,spurti-chopra/travis-cookbooks,dracos/travis-cookbooks,dracos/travis-cookbooks,dracos/travis-cookbooks,dracos/travis-cookbooks,travis-ci/travis-cookbooks,ljharb/travis-cookbooks,spurti-chopra/travis-cookbooks,ljharb/travis-cookbooks,travis-ci/travis-cookbooks