Add warning about Vagrant package in Ubuntu
Add warning about Vagrant package in Ubuntu

Follow up to https://github.com/freedomofpress/securedrop/pull/932#issuecomment-85165231

jeann2013/securedrop,ageis/securedrop,chadmiller/securedrop,jeann2013/securedrop,jaseg/securedrop,chadmiller/securedrop,ageis/securedrop,chadmiller/securedrop,chadmiller/securedrop,GabeIsman/securedrop,jrosco/securedrop,GabeIsman/securedrop,chadmiller/securedrop,conorsch/securedrop,ageis/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,garrettr/securedrop,conorsch/securedrop,kelcecil/securedrop,jaseg/securedrop,pwplus/securedrop,pwplus/securedrop,pwplus/securedrop,kelcecil/securedrop,micahflee/securedrop,conorsch/securedrop,kelcecil/securedrop,jrosco/securedrop,heartsucker/securedrop,chadmiller/securedrop,jeann2013/securedrop,ehartsuyker/securedrop,jeann2013/securedrop,conorsch/securedrop,jrosco/securedrop,jeann2013/securedrop,garrettr/securedrop,jaseg/securedrop,ehartsuyker/securedrop,jrosco/securedrop,heartsucker/securedrop,kelcecil/securedrop,kelcecil/securedrop,pwplus/securedrop,kelcecil/securedrop,jaseg/securedrop,jrosco/securedrop,pwplus/securedrop,ehartsuyker/securedrop,GabeIsman/securedrop,micahflee/securedrop,ageis/securedrop,heartsucker/securedrop,heartsucker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,GabeIsman/securedrop,conorsch/securedrop,micahflee/securedrop,jeann2013/securedrop,garrettr/securedrop,GabeIsman/securedrop,ehartsuyker/securedrop,micahflee/securedrop,GabeIsman/securedrop,jaseg/securedrop,jaseg/securedrop,jrosco/securedrop,pwplus/securedrop