GDAXExchangeTest: return promise instead of using done. (coinbase#229)

If an assertion inside a .then() block fails then mocha will timeout
instead of printing the actual error. So return the Promise instead so
mocha can print the actual error.

Example generated error when an assertion failes:

  1) GDAX Exchange API returns candles:
     Error: Timeout of 2000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.
      at Timeout.<anonymous> (node_modules/mocha/lib/runnable.js:232:19)
      at ontimeout (timers.js:466:11)
      at tryOnTimeout (timers.js:304:5)
      at Timer.listOnTimeout (timers.js:267:5)

test/exchanges/GDAXExchangeTest.ts

@@ -57,7 +57,7 @@ describe('GDAX Exchange API', () => {
         });
     });
 
-    it('loads the orderbook', (done) => {
+    it('loads the orderbook', () => {
         nock('https://api.gdax.com', {encodedQueryParams: true})
             .get('/products/BTC-USD/book')
             .query({level: '3'})
@@ -66,16 +66,15 @@ describe('GDAX Exchange API', () => {
                 bids: [['240', '5', 1], ['245', '3', 1], ['248', '1', 1]],
                 asks: [['250', '3', 1], ['251', '1.15', 1]]
             });
-        gdax.loadOrderbook('BTC-USD').then((book) => {
+        return gdax.loadOrderbook('BTC-USD').then((book) => {
             assert.equal(book.numBids, 3);
             assert.equal(book.numAsks, 2);
             assert(book.asksTotal.eq(4.15));
             assert(book.bidsTotal.eq(9));
-            done();
         });
     });
 
-    it('returns candles', (done) => {
+    it('returns candles', () => {
         nock('https://api.gdax.com', {encodedQueryParams: true})
             .get('/products/BTC-USD/candles')
             .query({granularity: '3600', limit: 3})
@@ -84,15 +83,14 @@ describe('GDAX Exchange API', () => {
                 [1519037580, 10911.81, 10924.99, 10924.99, 10911.82, 2.32705733],
                 [1519037520, 10910.33, 10925, 10910.33, 10924.99, 0.47783237999999995]
             ]);
-        gdax.loadCandles({gdaxProduct: 'BTC-USD', interval: '1h', limit: 3, from: null, extra: null}).then((candles: Candle[]) => {
+        return gdax.loadCandles({gdaxProduct: 'BTC-USD', interval: '1h', limit: 3, from: null, extra: null}).then((candles: Candle[]) => {
             assert.equal(candles.length, 3);
             assert.equal(candles[0].timestamp.valueOf(), 1519037640000);
             assert.equal(candles[0].open.toFixed(2), 10911.82);
             assert.equal(candles[1].high.toFixed(2), 10924.99);
             assert.equal(candles[2].low.toFixed(2), 10910.33);
             assert.equal(candles[0].close.toFixed(2), 10911.82);
             assert.equal(candles[1].volume.toFixed(2), 2.33);
-            done();
         });
     });
 });