[flamegraph.pl] Update to latest version

Since it seems brendangregg/FlameGraph doesn't do a regular release:

    https://github.com/brendangregg/FlameGraph/releases/tag/v1.0

I just used the most recent version of master by running the following
to update the script file:

    curl https://raw.githubusercontent.com/brendangregg/FlameGraph/1b1c6dee/flamegraph.pl > vendor/FlameGraph/flamegraph.pl