Update version and date for 3.1.

wiki/src/inc/stable_amd64_date.html

-2017-07-05
\ No newline at end of file
+2017-08-08
\ No newline at end of file

wiki/src/inc/stable_amd64_gpg_verify.html

-gpg --keyid-format 0xlong --verify tails-amd64-3.0.1.iso.sig tails-amd64-3.0.1.iso
+gpg --keyid-format 0xlong --verify tails-amd64-3.1.iso.sig tails-amd64-3.1.iso

wiki/src/inc/stable_amd64_iso_sig_url.html

-https://tails.boum.org/torrents/files/tails-amd64-3.0.1.iso.sig
+https://tails.boum.org/torrents/files/tails-amd64-3.1.iso.sig

wiki/src/inc/stable_amd64_iso_url.html

-http://dl.amnesia.boum.org/tails/stable/tails-amd64-3.0.1/tails-amd64-3.0.1.iso
+http://dl.amnesia.boum.org/tails/stable/tails-amd64-3.1/tails-amd64-3.1.iso

wiki/src/inc/stable_amd64_torrent_url.html

-https://tails.boum.org/torrents/files/tails-amd64-3.0.1.torrent
+https://tails.boum.org/torrents/files/tails-amd64-3.1.torrent

wiki/src/inc/stable_amd64_version.html

-3.0.1
+3.1