Automate the build of this gem

[flbulgarelli]

We should automatically run build - or at least, make build simpler - so that releasing mulang versions is straighforward. Something like this:

./bin/pull_mulang.sh && bundle exec rspec && \
git add lib/mulang/version.rb && \
git commit -m "Welcome $VERSION!" && \
git tag v$VERSION && \ 
gem build mulang.gemspec && \ 
gem push mulang-$VERSION.gem && \
git push origin HEAD --tags

[flbulgarelli]

We should have a look at https://github.com/mumuki/mumuki-styles @fedescarpa