Add postgis extension for postgresql

[igorlima]

This extension is needed for project that works with Spatial SQL. http://postgis.net/install/

[gmcmicken]

Yes please, finding it difficult to install from source...

[ijobling]

@gmcmicken If you are looking for this quickly, have you seen you can create your own box part for this and send us a PR - we will be able to get this merged in very quickly if you can do this See https://codio.com/docs/boxes/installsw/parts-coding/