update gem2arch to follow PKGBUILD-rubygem.proto

- directories created by gem2arch follow ruby guidelines (ruby-$_gemname)
- the PKGBUILD created by gem2arch define $_gemname
- all calls of '% (gem_spec.name)' are replaced by $_gemname

Signed-off-by: Andrwe Lord Weber <github@andrwe.org>
Merge pull request #49 from Andrwe/master

update gem2arch to follow PKGBUILD-rubygem.proto