send-to-fpaste: use /usr/bin/env instead of /bin/env

In light of the larger /usr - movement, it would be nicer to use
/usr/bin/env instead of /bin/env. Some distros do not offer /bin/env
(and never have). All other files refering to 'env' look for it in
/usr/bin as well, so it's just consistent.

https://bugzilla.gnome.org/show_bug.cgi?id=737931