I like your game, i do have some pros and cons for you.
Pros, Graphics, sounds are good, and the ship is quiet responsive which is great, good speed.
cons, i just moved in one direction away from the enemies, and then because im too fast, they never cought up and i could technically go on forever, i actually left your background image.
A way id improve that, is to add a boundary zone, which will do damage if you enter it, and also not make the enemy ships follow in a direct line, let them avoid eachother, and shoot you.