I use drone.io hosted drone. I wonder why documentation lacks basic information like OS used there, which is Ubuntu 12.04.2 LTS (Precise Pangolin) atm, at least in C/C++ variant (is it different for others?). I had to do some test builds to discover it.
In Travis-CI such information can be easily found in:
P.S. Because drone.io uses LTS version, I believe it should be upgraded to the latest version, which is 12.04.5.