Bump version to v0.5.0-pre
Bump version to v0.5.0-pre

arthurprs/mio,greensnark/mio,yuikns/mio,carllerche/mio,fuchsia-mirror/third_party-mio,carllerche/mio,alexcrichton/mio,geofft/mio,marjakm/mio,mlalic/mio,asomers/mio,rryan/mio,asomers/mio,tailhook/mio,dpc/mio,bottompawn/mio,mneumann/mio,fuchsia-mirror/third_party-mio,echochamber/mio,dwrensha/mio,frewsxcv/mio,murarth/mio,tempbottle/mio,fhartwig/mio,zslayton/mio,luca-barbieri/mio,danburkert/mio,rrichardson/mio,yangxingpping/mio,mneumann/mio