Update to latest rebar 2.4.0

When checking out cberl and compiling I encountered

```
 All 17 tests passed.
ERROR: Cover analyze failed for cberl_transcoder_tests: {not_cover_compiled, │
cberl_transcoder_tests} "...cberl_transcoder_tests.beam"
ERROR: Cover analyze failed for cberl_tests: {not_cover_compiled,cberl_tests}
"...cberl_tests.beam"

Cover analysis: /Users/phil/Source/cberl/.eunit/index.html
```

This bug was fixed in rebar by https://github.com/basho/rebar/issues/310 so
updating to a later rebar gets rid of this, no other changes were needed to
make the new rebar work so I think it's worth upgrading.