travis: remove unneeded Go version 'release'
travis: remove unneeded Go version 'release'

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