Test case for USD-LTC pair

[karma0]

There is a pair (USD-LTC) missing from the list in the following reference, causing unit tests to fail.

python-bittrex/bittrex/test/bittrex_tests.py

Line 75 in 2dbc08e

self.assertListEqual(['BTC-LTC', 'ETH-LTC', 'USDT-LTC'], actual)