Various fixes

Here is a pull request with fixes for the various problems I encountered while using activeadmin-axlsx. Here are the details :

- First I had to upgrade Gemfile to work with my ActiveAdmin version that I directly fetched from github, the gem release being quite old.
- I had to tweak railtie initialization to avoid an error occurring when loading Rails application environment. I had to tweak it so it doesn't break my custom helpers. (cf. https://github.com/randym/activeadmin-axlsx/pull/19)
- I had to apply a fix to allow defining custom columns (https://github.com/randym/activeadmin-axlsx/pull/33)
- I updated initializer to avoid database hit while initializing ActiveAdmin