HELIO-3668 fix failing travis ci builds
HELIO-3668 fix failing travis ci builds

travis ci builds are failing with libv8 compilation errors.
It might be because bundler 2.2 was released yesterday and we've
been installing bundler on travis with "--pre".

Something seems to be up with bundler 2.2 being "smart" about fetching
pre-built gems. Maybe.

https://github.com/rubyjs/libv8/issues/310#issuecomment-742481001

mlibrary/heliotrope,mlibrary/heliotrope,mlibrary/heliotrope,mlibrary/heliotrope