bundlerEnv: Add option to copy gem files instead of symlinking

The way ruby loads gems and keeps track of their paths seems to not
always work very well when the gems are accessed through
symlinks. Ruby will then complain that the same files are loaded
multiple times; it relies on the file's full path to determine whether
the file is loaded or not.

This adds an option to simply copy all gem files into the environment
instead, which gets rid of this issue, but may instead result in major
file duplication.