Pin Ruby buildpacks - CF v1.7.40, Heroku v200 (fixes #147)

[textbook]

• A short explanation of the proposed change:

  Switch the pinned Ruby buildpack for CF deployments to v1.7.40 and the Ruby buildpack for Heroku deployments to v200

• An explanation of the use cases your change solves

  Fixes error on deployment caused by mismatched versions of bundler - "Could not find 'bundler'" on PCF deployment #147

• Links to any other associated PRs

  N/A

• I have reviewed the contributing guide

• I have made this pull request to the master branch

• I have run all the tests using ./test.sh - no change to code, I tested the deployments to Heroku and PCFOne manually

• I have added the copyright headers to each new file added - N/A

• I have given myself credit in the humans.txt file (assuming I want to) - already done

[tom025]

Would it be better to update bundler for the project with bundle update --bundler rather than dropping the buildpack version?

[butzopower]

@tom025 Do you mean run that command locally to update the BUNDLED WITH in Gemfile.lock or do you mean run that command when the buildpack runs?

[tom025]

Yes bundle update --bundler should update the Gemfile.lock.

Given that the heroku build pack has not been pinned yet and that is the more pressing issue I will merge this now. I will also open an issue for updating bundler/buildpack versions.