Fetch nvm-exec
Fetch nvm-exec

See https://github.com/travis-ci/travis-build/pull/971/files#r104523288

craigcitro/travis-build,JuliaCI/travis-build,kevmoo/travis-build,kevmoo/travis-build,craigcitro/travis-build,andyli/travis-build,andyli/travis-build,JuliaCI/travis-build