Emulate the Travis method of calling travis_retry

As per the Travis build script, use eval() for the content passed to travis_retry():
* https://github.com/travis-ci/travis-build/blob/8cf85e05/lib/travis/build/script/templates/header.sh#L39
Merge pull request #76 from sgerrand/travis-retry-needs-eval

Emulate the Travis method of calling travis_retry