handle ruby-build error when attempting to run rbenv install

[travi]

{ Error: Command failed with exit code 2 (ENOENT): rbenv install --skip-existing
    at makeError (~/.nvm/versions/node/v10.16.3/lib/node_modules/@travi/cli/node_modules/execa/lib/error.js:58:11)
    at handlePromise (~/.nvm/versions/node/v10.16.3/lib/node_modules/@travi/cli/node_modules/execa/index.js:112:26)
    at process._tickCallback (internal/process/next_tick.js:68:7)
  command: 'rbenv install --skip-existing',
  exitCode: 2,
  exitCodeName: 'ENOENT',
  stdout: '',
  stderr:
   'ruby-build: definition not found: 2.6.3\n\nSee all available versions with `rbenv install --list\'.\n\nIf the version you need is missing, try upgrading ruby-build:\n\n  brew update && brew upgrade ruby-build',
  all:
   'ruby-build: definition not found: 2.6.3\n\nSee all available versions with `rbenv install --list\'.\n\nIf the version you need is missing, try upgrading ruby-build:\n\n  brew update && brew upgrade ruby-build',
  failed: true,
  timedOut: false,
  isCanceled: false,
  killed: false,
  signal: undefined }