wrk: improve test.

`-d 1` makes `wrk` only run for 1 second, which is the least amount of time. The default is 10 seconds. And we'll hit `example.com` instead of GitHub since example.com fronted by a CDN and whatnot.
wrk: add 3.1.0 bottle.

Closes #30966.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
libusb 1.0.19
libusb: update 1.0.19 bottle.

Closes #30953.
gcc: 4.9.1

Version bump gcc from 4.8.x to 4.9.1 on @jacknagel’s
[suggestion](https://github.com/Homebrew/homebrew/pull/30872#issuecommen
t-49511203).

Following the discussion I’ve also killed the ``` HEAD ```, but if
Mike/Jack/Adam/Misty/etc want that back in I’ll submit another commit
to re-insert it.
gcc: update 4.9.1_1 bottle.