Removed support for yes/no values in favor of true/false.

Travis is giving this errors couse of this.
OpenRA.Lint(1,1): Error: ftnk.ExplodesInfo.Weapon: Missing weapon `FlametankExplode`.
OpenRA.Lint(1,1): Error: ftnk.ExplodesInfo.EmptyWeapon: Missing weapon `FlametankExplode`.