Address Travis issues with Bundler failures
Address Travis issues with Bundler failures

- This issue: https://github.com/bundler/bundler/issues/3558
- PASS: Gem version 2.5.1 & Bundler 1.11.1
- FAIL: Gem version 2.5.1 & Bundler 1.7.6
- PASS: Gem version 2.4.3 & Bundler 1.7.6

dimko/pdfkit,pdfkit/pdfkit,ContinuityControl/pdfkit