Rename Flak Truck to Mobile Flak

Renamed Flak Truck to Mobile Flak, also updated the unit to use a new icon provided by @KatzSmile, located here: http://www.sleipnirstuff.com/forum/viewtopic.php?f=83&p=286374#286374. Matches the unit in game more closely as well.

Comparison of old (left) + new (right):
![flakicon2](https://f.cloud.github.com/assets/566742/2354402/f14146d8-a5be-11e3-8790-07c483e26ec3.png)