Upgrade ReSpec to commit 1258dea247b803c48e1c52372f9f4a07c487d1c0

This ReSpec commit got rid of the button in the static version of the
spec. Unfortunately, the commit immediately following it broke the
respec2html.js script, so we cannot upgrde and further until that is
fixed: https://github.com/darobin/respec/pull/294