BAU: update Ruby

[robinwhittleton]

2.3.1 release notes: http://svn.ruby-lang.org/repos/ruby/tags/v2_3_1/ChangeLog

(also update libv8 to fix compilation on Clang 3.7.0 / OS X 10.11.4+)

[robinwhittleton]

One thing that needs to be discussed: Gemfile.lock still specifies a ruby version of 2.3.0. I dug into why this wasn’t being updated and found this issue filed against bundler, which references this PR that unfortunately isn’t in a release version at the moment. Locally, rbenv uses the version defined in .ruby-version but I’m not sure what will happen on staging/production.

[chrisholmes]

I've encountered a few problems when trying to build this using our packager job. I'll try and have a closer look later, but we'll need to hold on merging.

[lloydnye]

@chrisholmes does your latest commit (use heroku buildpack directly) solve the packager issue you mentioned?

[chrisholmes]

No, unfortunately. I'm going to be raising a story to address this problem formally.

I'm going to be closing this now if that's OK.