Add safety checks to promote-nightly-to-release.sh

[fredemmott]

• make sure date is recent (e.g. last 7 days)
• compare to current hhvm tags - if 4.x is requested, make sure that 4.x.0 does not already exist, and that 4.(x-1).0 does
• make sure that we have gnu sed available (SED="$(if [ "$(uname -s)" == "Darwin" ]; then echo gsed; else echo sed; fi)" then checking that $SED is executable)

This could hard-fail (and be commented out to override) or prompt for confirmation (not for sed failure though)

[fredemmott]

Also #207 to check remote