Browser.wait() until .getAttribute() returns true

I'm still learning Protractor so I'm not sure if this is a simple answer that I'm not getting but I'm just trying to have the browser wait until the attribute I'm retrieving is true.

I was trying something like this:

browser.wait(function() {
    topping.getAttribute('aria-expanded').then(function(value) {
        if(value == 'true') {
            return true;
        };
    });
}, 5000);

Since .getAttribute() only returns as a string or null I tried to use an if statement to return an actual true value. This doesn't appear to work. Am I misunderstanding the .then() function or maybe misunderstanding browser.wait()?

Complete code:

browser.get('http://ift.tt/1GgQA5W');

var topping = element(by.model('topping'));

topping.click();

browser.wait(function() {
    return topping.getAttribute('aria-expanded').then(function(value) {
        return value == 'true';
    });
}, 5000);

var toppingOptions = element.all(by.css('[role="option"]'));

toppingOptions.get(5).click();

expect(topping.getText()).toBe('Onion');

One more note, if I put browser.sleep(1000); after topping.click() and before browser.wait() then the test passes. So I know the rest of the test isn't what's failing. For some reason the call to wait isn't working.