Hey Notmuchers,

I'm running the latest NixOS and noticed that the system does not
install gpgv1 by default. This means that the only gpg binary available
is the gpg2 binary.

I found that notmuch-crypto.el hardcodes the GnuPG binary as gpg.  I
thought it might make more sense to rely on the setting provided by 
EasyPG to locate the correct GnuPG binary.

The patch is quite simple - it just replaces "gpg" with the
epg-gpg-program variable in each place it's used.

-	(call-process "gpg" nil t t "--list-keys" fingerprint))
+	(call-process epg-gpg-program nil t t "--list-keys" fingerprint))

I tested the changes locally and it seems to work OK. 

Happy New Year!
John