require bundler 1.5.1

to get ruby 2.1 as a platform for the gemfile
also make the gemfile stricter (and more helpful) on bundler version
mismatch (it used to not check the bundler version requirement if
the rest of the bundle checked out)

Change-Id: I1cc5122f3b2f0ea212a5cfd2259308026efb0f50
Reviewed-on: https://gerrit.instructure.com/27891
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>