Remove dead links.
Remove dead links.
Merge pull request #308 from YuzuruS/remove-dead-links

Remove dead links
add elixir development box
resolve URL to dropboxusercontent 

Change box URL to resolve through dropboxusercontent
Merge pull request #310 from rpepato/master

Add elixir development box
Add CentOS7 Box
Amended url
Merge pull request #311 from wingZero21/patch-1

Add CentOS7 Box