[website] Fix the bundler gem installation

Summary: The current version of bundler is no longer compatible with our old ruby version, so we have to manually specify the bundler version to install.

Test Plan:

docker build -t website .

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15045