build: print a warning on go version 1.5.{1,2}
build: print a warning on go version 1.5.{1,2}

Ubuntu does not have golang 1.5.3 yet, so the check was completely
blocking the build on Ubuntu. Replace it with a warning.

Related to https://github.com/coreos/rkt/pull/2006

ybubnov/rkt,mike1729/rkt,Wilybald/rkt,sstabellini/rkt,squeed/rkt,blixtra/rkt,il9ue/rkt,yifan-gu/rkt,Shashwatsh/rkt,dgonyeo/rkt,tmrts/rkt,shobhit85/rkt,kinvolk/rkt,yifan-gu/rkt,jonboulle/rkt,markllama/rkt,dgonyeo/rkt,achanda/rkt,Wilybald/rkt,achanda/rkt,alban/rkt,yifan-gu/rkt,jakon89/rkt,sgotti/rkt,jjlakis/rkt,s-urbaniak/rkt,rkt/rkt,nhlfr/rkt,rkt/rkt,dgonyeo/rkt,monstermunchkin/rkt,joshix/rkt,intelsdi-x/rkt,endocode/rkt,sinfomicien/rkt,intelsdi-x/rkt,yifan-gu/rocket,sinfomicien/rkt,il9ue/rkt,sstabellini/rkt,shobhit85/rkt,sgotti/rkt,squeed/rkt,matthaias/rkt,kinvolk/rkt,rkt/rkt,yifan-gu/rocket,s-urbaniak/rkt,akshaykarle/rkt,akshaykarle/rkt,lucasallan/rkt,monstermunchkin/rkt,ybubnov/rkt,Wilybald/rkt,endocode/rkt,euank/rkt,cab105/rkt,blixtra/rkt,cab105/rkt,kinvolk/rkt,monstermunchkin/rkt,coreos/rkt,alban/rkt,ybubnov/rkt,Shashwatsh/rkt,blixtra/rkt,sinfomicien/rkt,s-urbaniak/rkt,coreos/rkt,rkt/rkt,cab105/rkt,il9ue/rkt,mike1729/rkt,alban/rkt,nhlfr/rkt,akshaykarle/rkt,markllama/rkt,Shashwatsh/rkt,euank/rkt,joshix/rkt,endocode/rkt,shobhit85/rkt,jonboulle/rkt,squeed/rkt,jjlakis/rkt,kinvolk/rkt,markllama/rkt,lucasallan/rkt,tmrts/rkt,jjlakis/rkt,nhlfr/rkt,sgotti/rkt,lucasallan/rkt,jakon89/rkt,joshix/rkt,sstabellini/rkt,matthaias/rkt,euank/rkt,achanda/rkt,jakon89/rkt,tmrts/rkt,yifan-gu/rocket,mike1729/rkt,matthaias/rkt,jonboulle/rkt,coreos/rkt,intelsdi-x/rkt,Wilybald/rkt,joshix/rkt,lucasallan/rkt