fix casting error on numpy 1.11.0 when loading brain vision data

[jscastanoc]

the previous version was triggering the error:

TypeError: Cannot cast ufunc multiply output from dtype('float64') to dtype('int64') with casting rule 'same_kind'

explicit casting is necessary to about this

[venthur]

Hi @jscastanoc thank you for the fix. I've merged it and pushed it it wyrm's new home for future pull requests please use the bbci repository instead, as this one here is deprecated.

Thanks!