Fixing bundler on ruby-head build

[kpumuk]

One of the constants in Pathname was marked as private. The constants is used in all Bundler versions, and only landed on master and is not released as of today (4.0.11 is the current version). This makes ruby-head incompatible with any Bundler released to date.

This change switched ruby-head to use bundled Bundler instead of the version locked in our Gemfile.lock.

I personally don't like this and my best guess is that the change will be reverted (or made backwards-compatible), as the scale is large, but for now this MR fixed Thrift builds.

There is an active discussion going on to deprecate instead of breaking things, will monitor.

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.