tl,dr: Ubuntu versions 12.04 and 14.04 are here to stay. 14.10 ends beginning this summer. As such, 12.04 and 14.04 are the versions i will be developing for.
The long version:
Versions 12.10, 13.04 and 13.10 have reached their end of life date meaning the main repositories for them are not available and there is no longer any support for them. This means supporting them in my project is a futile effort at best.
Version 12.04.5 is a long term support release meaning it will have its repositories intact and be supported by its developers until 2017. As such I am definitely planning on making sure my project works with it.
The latest version of 14.04 is supported until 2016. As such, I will mainly be developing my project on it form now on.
Version 14.10 is only supported until July of this year, just less than 4 months away. As such I will be checking that things work on it but not be developing for it.