changed playbook name
changed playbook name

The install.md directions is still using 'site.yml'. Until @conor does
the ansible re-org we should just change the directions to point to the
`securedrop-prod.yml` playbook.

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