Upgrading Bundler Version on Heroku - ruby-on-rails

Question

I need an updated version of bundler on my heroku application (cedar 14) and I haven't seen any reason why I can't do this. I'm stuck at 1.6.6 which is a couple months old and I need the most up to date version.
I need to specify a source: option for one of my gems and the older version of bundler doesn't allow this. Additionally, running heroku run gem update bundler successfully updates but 'heroku run bundle -v' yields 1.6.6.
Why?

Answer 1

You can modify the bundler version used by specifying the Ruby Buildpack on Heroku:
If you see that a different version of Bundler is being used by your
application than is listed in the Ruby Support article, your
application might be configured to use the master branch of the Ruby
buildpack for deployments.
To check which buildpack is configured, use the heroku buildpacks
command:
> heroku buildpacks
> === hidden-temple-25627 Buildpack URL https://github.com/heroku/heroku-buildpack-ruby.git
The example output shows that an unreleased version of the buildpack
is being used. To switch to the supported version, set the buildpack
as follows:
heroku buildpacks:set heroku/ruby Buildpack set. Next release on
hidden-temple-25627 will use heroku/ruby. Run git push heroku master
to create a new release using this buildpack.
You can verify that the new buildpack has been set by typing:
> heroku buildpacks
> === hidden-temple-25627 Buildpack URL heroku/ruby
Another reason your app might not be using the currently supported
Bundler version is if it is configured to deploy using a different
buildpack URL. This will happen if the BUILDPACK_URL config var is
set.
> heroku config:get BUILDPACK_URL BUILDPACK_URL:
> https://github.com/heroku/heroku-buildpack-ruby.git
If you see any value, then you are using a custom buildpack. If this
value is set to a “multi buildpack” such as
https://github.com/heroku/heroku-buildpack-multi then you will need to
check the .buildpacks file to see what buildpacks are used in
deployment. If you are using this method of deployment we recommend
that you instead follow the guidance in the Using Multiple Buildpacks
for an App article.
The officially deployed Ruby buildpack will sometimes lag behind
master for a few days.
Other than that it can not be directly configured:
Why can’t the Bundler version be configured?
Different versions of Bundler have different known bugs and slightly
different behavior. The Bundler version on Heroku is carefully
curated. A balance needs to be struck between supporting new Bundler
features and stability. The work we put into curating the Bundler
version ensures maximum stability, and avoids deprecation and
notification cycles on Bundler as it changes, or as bugs are fixed or
security issues are patched by Heroku.
The Ruby experience on Heroku is provided by the Heroku Ruby
Buildpack. This is the tool that installs a version of Bundler and
runs all the commands needed to get your application set up. The
buildpack relies on publicly exposed internals of Bundler. It is
tested and known to work against the currently specified version of
Bundler. If you were to take a modern buildpack and use it with an
older version of bundler, you would be likely to see unpredictable
results.
https://devcenter.heroku.com/articles/bundler-version