The Pigs Can Fly command now works because I fixed it today. It used to place a repeating command block that would summon the pigs riding the bats, but that didn't work, so now this command simply summons a pig riding a bat. Put command in a repeating command block (if you are playing 1.9) or use redstone clock circuit (if you are playing 1.8.9 or anything lower) for constant summon. The bats are also invisible now (particles hidden) so it gives you the impression that the pig is flying on its own.
And, if you are playing 1.8.9-, use this command instead:
/summon Pig ~ ~1 ~ {Riding:{id:"Bat",ActiveEffects:[{Id:14,Amplifier:1,Duration:10000000,ShowParticles:0b}]}}