scripts/vagrant: use ubuntu 15.04
scripts/vagrant: use ubuntu 15.04

Ubuntu 14.04 has some problems with incorrectly set up cgroups (#1042).

krnowak/rkt,yifan-gu/rkt,kinvolk/rkt,adfernandes/rkt,markllama/rkt,jzelinskie/rkt,linki/rkt,akshaykarle/rkt,blixtra/rkt,kbrwn/rkt,dalanlan/rkt,matthaias/rkt,Wilybald/rkt,tmrts/rkt,MattDees/rkt,intelsdi-x/rkt,dgonyeo/rkt,tv42/rkt,squaremo/rkt,monstermunchkin/rkt,sstabellini/rkt,yifan-gu/rkt,iaguis/rkt,cab105/rkt,achanda/rkt,luebken/rkt,adfernandes/rkt,jakon89/rkt,s-urbaniak/rkt,achanda/rkt,matthaias/rkt,tmrts/rkt,cab105/rkt,alexhersh/rkt,sstabellini/rkt,yifan-gu/rocket,cab105/rkt,euank/rkt,il9ue/rkt,iankronquist/rkt,shobhit85/rkt,liugenping/rkt,MattDees/rkt,mjg59/rkt,Shashwatsh/rkt,kbrwn/rkt,liugenping/rkt,jasonchaffee/rkt,nhlfr/rkt,Wilybald/rkt,sstabellini/rkt,mike1729/rkt,fdserr/rkt,alexhersh/rkt,endocode/rkt,squeed/rkt,luebken/rkt,linki/rkt,smarvin/rkt,jakon89/rkt,coreos/rkt,Yhgenomics/rkt,Wilybald/rkt,akshaykarle/rkt,marineam/rkt,krnowak/rkt,fdserr/rkt,endocode/rkt,Wilybald/rkt,kinvolk/rkt,squeed/rkt,intfrr/rkt,euank/rkt,squeed/rkt,smarvin/rkt,fdserr/rkt,yifan-gu/rocket,dalanlan/rkt,kinvolk/rkt,alexhersh/rkt,rhvgoyal/rkt,sinfomicien/rkt,rkt/rkt,squaremo/rkt,jonboulle/rkt,iaguis/rkt,jjlakis/rkt,sgotti/rkt,sinfomicien/rkt,jzelinskie/rkt,jonboulle/rkt,marineam/rkt,s-urbaniak/rkt,iankronquist/rkt,euank/rkt,alban/rkt,jasonchaffee/rkt,jjlakis/rkt,markllama/rkt,lucasallan/rkt,mjg59/rkt,liubq919/rkt,rkt/rkt,nhlfr/rkt,intelsdi-x/rkt,ybubnov/rkt,lucasallan/rkt,krnowak/rkt,lee212/rkt,tv42/rkt,dgonyeo/rkt,mike1729/rkt,ChengTiesheng/rkt,jakon89/rkt,kbrwn/rkt,Shashwatsh/rkt,monstermunchkin/rkt,markllama/rkt,ybubnov/rkt,liubq919/rkt,shobhit85/rkt,maquanyi/rkt,matthaias/rkt,Yhgenomics/rkt,adfernandes/rkt,akshaykarle/rkt,nhlfr/rkt,jzelinskie/rkt,rhvgoyal/rkt,luebken/rkt,s-urbaniak/rkt,chrisseto/rkt,kinvolk/rkt,achanda/rkt,dgonyeo/rkt,sgotti/rkt,marineam/rkt,joshix/rkt,monstermunchkin/rkt,jonboulle/rkt,squaremo/rkt,iankronquist/rkt,MattDees/rkt,intfrr/rkt,lucasallan/rkt,Yhgenomics/rkt,mjg59/rkt,endocode/rkt,ChengTiesheng/rkt,maquanyi/rkt,intelsdi-x/rkt,chrisseto/rkt,tv42/rkt,alban/rkt,sinfomicien/rkt,jjlakis/rkt,joshix/rkt,lee212/rkt,smarvin/rkt,ChengTiesheng/rkt,tmrts/rkt,rkt/rkt,alban/rkt,ybubnov/rkt,maquanyi/rkt,shobhit85/rkt,il9ue/rkt,Shashwatsh/rkt,intfrr/rkt,il9ue/rkt,rkt/rkt,mike1729/rkt,dalanlan/rkt,blixtra/rkt,liugenping/rkt,sgotti/rkt,liubq919/rkt,iaguis/rkt,lee212/rkt,yifan-gu/rocket,joshix/rkt,lucasallan/rkt,yifan-gu/rkt,coreos/rkt,jasonchaffee/rkt,blixtra/rkt,chrisseto/rkt,coreos/rkt,linki/rkt,joshix/rkt,rhvgoyal/rkt