Retrying travis-artifact gem install.

This gem installation has been chronically failing. After raising https://github.com/travis-ci/travis-artifacts/issues/22, I was instructed to try this fixe. Perhaps it will help.
Merge pull request #542 from rhyolight/travis-retry-artifact-install

Retrying travis-artifact gem install.
Merge branch 'master' of https://github.com/numenta/nupic