readme: bump rocket version to 0.3.1
readme: bump rocket version to 0.3.1

This updates the recommended rocket install to the latest release. Thanks!
sstabellini/rkt,dalanlan/rkt,sgotti/rkt,kinvolk/rkt,s-urbaniak/rkt,jjlakis/rkt,coreos/rkt,lee212/rkt,intfrr/rkt,ramitsurana05/rkt,lsm5/rkt,squeed/rkt,ramitsurana05/rkt,blixtra/rkt,shtouff/rkt,jasonchaffee/rkt,michelangelo13/rkt,cab105/rkt,shtouff/rkt,xnox/rkt,marineam/rkt,Wilybald/rkt,MattDees/rkt,iankronquist/rkt,mattthias/pkg-rkt,alban/rkt,smarvin/rkt,MabinGo/rkt,tv42/rkt,alexhersh/rkt,michelangelo13/rkt,rhvgoyal/rkt,bsmr-go/rkt,tixxdz/rkt,clearlinux/rkt,MabinGo/rkt,tixxdz/rkt,joshix/rkt,liubq919/rkt,dgonyeo/rkt,shobhit85/rkt,monstermunchkin/rkt,shtouff/rkt,crawford/rkt,jzelinskie/rkt,iaguis/rkt,ybubnov/rkt,yifan-gu/rocket,linki/rkt,lucasallan/rkt,yifan-gu/rkt,achanda/rkt,intfrr/rkt,adfernandes/rkt,akshaykarle/rkt,liubq919/rkt,bsmr-go/rkt,Shashwatsh/rkt,coreos/rkt,eyakubovich/rkt,smarvin/rkt,rhvgoyal/rkt,blixtra/rkt,matthaias/rkt,crawford/rkt,endocode/rkt,ybubnov/rkt,jjlakis/rkt,squeed/rkt,euank/rkt,s-urbaniak/rkt,markllama/rkt,ramitsurana05/rkt,mike1729/rkt,il9ue/rkt,akshaykarle/rkt,yifan-gu/rocket,kbrwn/rkt,Wilybald/rkt,kinvolk/rkt,Wilybald/rkt,alban/rkt,intfrr/rkt,tixxdz/rkt,ChengTiesheng/rkt,ramitsurana/rkt,ChengTiesheng/rkt,liugenping/rkt,sstabellini/rkt,squeed/rkt,ChengTiesheng/rkt,jonboulle/rkt,akshaykarle/rkt,sgotti/rkt,cab105/rkt,lucasallan/rkt,luebken/rkt,sinfomicien/rkt,clearlinux/rkt,maquanyi/rkt,liugenping/rkt,krnowak/rkt,Yhgenomics/rkt,sinfomicien/rkt,klizhentas/rkt,jonboulle/rkt,matthaias/rkt,euank/rkt,il9ue/rkt,MattDees/rkt,chrisseto/rkt,jasonchaffee/rkt,wparad/rkt,mike1729/rkt,sstabellini/rkt,intelsdi-x/rkt,eyakubovich/rkt,mike1729/rkt,rhvgoyal/rkt,luebken/rkt,lucasallan/rkt,linki/rkt,alexhersh/rkt,iaguis/rkt,rot26/rkt,MabinGo/rkt,fdserr/rkt,rot26/rkt,jzelinskie/rkt,fdserr/rkt,ramitsurana/rkt,clearlinux/rkt,intelsdi-x/rkt,chrisseto/rkt,maquanyi/rkt,rot26/rkt,dgonyeo/rkt,alban/rkt,ybubnov/rkt,kbrwn/rkt,jzelinskie/rkt,jakon89/rkt,markllama/rkt,Wilybald/rkt,Shashwatsh/rkt,nhlfr/rkt,joshix/rkt,nhlfr/rkt,krnowak/rkt,ramitsurana/rkt,xnox/rkt,tmrts/rkt,klizhentas/rkt,cab105/rkt,Shashwatsh/rkt,eyakubovich/rkt,matthaias/rkt,s-urbaniak/rkt,xnox/rkt,sinfomicien/rkt,alexhersh/rkt,liubq919/rkt,joshix/rkt,iankronquist/rkt,michelangelo13/rkt,adfernandes/rkt,jjlakis/rkt,mattthias/pkg-rkt,robszumski/rocket,crawford/rkt,Yhgenomics/rkt,lee212/rkt,iaguis/rkt,mjg59/rkt,wparad/rkt,kinvolk/rkt,euank/rkt,achanda/rkt,shobhit85/rkt,nhlfr/rkt,squaremo/rkt,coreos/rkt,markllama/rkt,endocode/rkt,jakon89/rkt,wparad/rkt,il9ue/rkt,blixtra/rkt,jakon89/rkt,klizhentas/rkt,kinvolk/rkt,tmrts/rkt,squaremo/rkt,rkt/rkt,lsm5/rkt,tv42/rkt,jasonchaffee/rkt,sgotti/rkt,Yhgenomics/rkt,rkt/rkt,rkt/rkt,monstermunchkin/rkt,lsm5/rkt,MattDees/rkt,chrisseto/rkt,dalanlan/rkt,shobhit85/rkt,mjg59/rkt,tmrts/rkt,robszumski/rocket,tv42/rkt,yifan-gu/rkt,linki/rkt,jonboulle/rkt,lucasallan/rkt,yifan-gu/rocket,monstermunchkin/rkt,dalanlan/rkt,smarvin/rkt,mjg59/rkt,maquanyi/rkt,liugenping/rkt,achanda/rkt,dgonyeo/rkt,kbrwn/rkt,intelsdi-x/rkt,krnowak/rkt,iankronquist/rkt,endocode/rkt,rkt/rkt,lee212/rkt,joshix/rkt,marineam/rkt,marineam/rkt,mattthias/pkg-rkt,squaremo/rkt,yifan-gu/rkt,bsmr-go/rkt,fdserr/rkt,adfernandes/rkt,robszumski/rocket,luebken/rkt