doc: now recommending ubuntu 14.04.5
doc: now recommending ubuntu 14.04.5

conorsch/securedrop,ageis/securedrop,micahflee/securedrop,garrettr/securedrop,micahflee/securedrop,ehartsuyker/securedrop,micahflee/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,garrettr/securedrop,heartsucker/securedrop,ageis/securedrop,garrettr/securedrop,conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop,conorsch/securedrop,ageis/securedrop,ageis/securedrop