CI: only run gox -build-toolchain for Go < 1.5
CI: only run gox -build-toolchain for Go < 1.5

mappu/restic,middelink/restic,kurin/restic,byo/restic,stuertz/restic,howeyc/restic,kurin/restic,ckemper67/restic,bchapuis/restic,middelink/restic,jayme-github/restic,ar-jan/restic,jayme-github/restic,fawick/restic,pombredanne/restic,stuertz/restic,arithmetric/restic,restic/restic,ar-jan/restic,episource/restic,byo/restic,restic/restic,fawick/restic