Bangladesh Currency code is BDT

This was updated in the currencies gem and needs to be updated here
https://github.com/hexorx/currencies/commit/2a95dd4692e0aad644c74801385b7b868aa70572
http://www.xe.com/iso4217.php#B
Merge pull request #75 from bradleypriest/patch-2

Bangladesh Currency code is BDT