Update vendored version of rebar3

[edmorley]

The version of rebar3 vendored in this repo gave this error using MSYS2 on Windows:

$ ./rebar3 ct
===> Verifying dependencies...
===> Linking _build/default/lib/cowboyku to _build/test/lib/cowboyku
===> Upgrading cowboyku ({git,
                                 "https://github.com/heroku/cowboyku.git",
                                 {ref,
                                  "--git-dir=c:/Users/Ed/src/_todo/vegur/_build/default/lib/cowboyku/.git\n--work-tree=c:/Users/Ed/src/_todo/vegur/_build/default/lib/cowboyku\n27ed003dcd81bcfea9f00f163534dd00e11386c5"}})
===> Failed to fetch and copy dep: {git,"https://github.com/heroku/cowboyku.git",
                                  {ref,
                                      "--git-dir=c:/Users/Ed/src/_todo/vegur/_build/default/lib/cowboyku/.git\n--work-tree=c:/Users/Ed/src/_todo/vegur/_build/default/lib/cowboyku\n27ed003dcd81bcfea9f00f163534dd00e11386c5"}}

Updating to a newer release (and blowing away the malformed changes to rebar.lock) fixed the problem.

To avoid confusion it would be great to either update the version of rebar3 vendored in this repo, or else remove it entirely and have the have the rebar3 installation instructions listed in the README. I'd do the former myself but you'd then have to verify the binary anyway, which would somewhat defeat the point :-)

Many thanks!