When we say travel makes you "smarter," we don't mean you're all of a sudden going to become a genius. (If that happens, let us know in the comments.) But traveling will certainly make you more worldly, force you to think in different ways and help you to embrace unique cultural practices. All of which will ultimately create more neurological connections in your brain, making you quicker to react, think through logic and work through problem solving more efficiently.
What Einstein said is at the root of what this brief discussion is about. You're only as smart as your experience, and if you don't challenge yourself past your comfort zone -- you're setting limits on your potential that need not be there. Both new experiences and doing things you already know -- the hard way -- will keep your cognitive skills sharp.