Specify utf-8 as the encoding for currencies.json

[cgeorgelee]

This is a fix for issue: "Attempt to read /raw_data/currencies.json as ascii instead of utf-8 breaks get_symbol()" - https://github.com/MicroPyramid/forex-python/issues/109

We tell the file reader the encoding of /raw_data/currencies.json instead of using python auto detection.