Use File.join instead of manual path creation.

This prevents issues, when File.join in #new_default_spec removes
superfluous slashes while they are kept in expected paths. E.g. the test
would fail if ruby configuration specifies --with-ruby-version=''.
Do not add last slash to Gem.user_dir if ruby_version string is empty.
Merge pull request #455 from voxik/fixes-for-empty-ruby-version

Fixes for configure --with-ruby-version=''