Fix base / quote volume parse
Fix base / quote volume parse

The format for ticker data from the Gemini API is:
```
{
    "ask": "977.59",
    "bid": "977.35",
    "last": "977.65",
    "volume": {
        "BTC": "2210.505328803",
        "USD": "2135477.463379586263",
        "timestamp": 1483018200000
    }
}
```
so this fixes fetching the market info as a string and not a float and
renames a variable to avoid that type of problem in the future.

ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt