Force copying files for mruby

When installing mruby-dev, the cp command doesn't use the force flag.
This causes an issue when using a custom build configuration (via
MRUBY_CONFIG environment variable) and installing mrbgems from Github,
as the pack files in the git repositories are read-only.
Recursive copying fixes and tests

Use the `cp -f` flag to force unlinking non-writable files.

Closes #384
Merge branch 'copy-recursive'