Fix Voice::Encoder volume option for avconv
Fix Voice::Encoder volume option for avconv

megumisonoda/discordrb,Roughsketch/discordrb,meew0/discordrb,VxJasonxV/discordrb,VxJasonxV/discordrb,Roughsketch/discordrb,sarsntony/discordrb,sarsntony/discordrb,meew0/discordrb,megumisonoda/discordrb