Rails 4 support

As of RoR 4 release is coming soon, gems should support it to make migration very easy. :-)

Good news: Rails 4 is supported by this gem out of the box, but gemspec forbids it's installation in rails 4 app. So I've fixed it.

As a bonus I've added testing for all supported rails versions (see Travis build for this pull request).

On a local machine:
```bash
rm Gemfile.lock 
rm spec/dummy/db/*.sqlite3 
export RAILS_VERSION=4.0
bundle install --binstubs
bundle exec spec/ci.sh
```

P.S> Thanks for a gem, that makes axlsx installation super easy!