Ensure the linux version of libv8-node gets installed

mini_racer depends on libv8-node, this uses a compiled binary. Bundler should automatically install the linux version but instead uses the linux-musl one.

There are a few issues documenting this behavior

https://github.com/rubyjs/mini_racer/issues/218 https://github.com/rubyjs/libv8-node/issues/5

Setting up the platform will make this usable under linux (ugly, but a non resolved issue will break deployment with ruby 2.5