Mruby is broken

I'm using mruby to test things in my local checkout of fuzzbench and it I noticed that it is currently broken as it changed from the version that I uploaded months ago.

+ The build script is ignoring the compiler wrappers overriding CC and CXX https://github.com/google/fuzzbench/blob/5d7362a6fc6647740abdebd0c7db13beb57809ea/benchmarks/mruby-2018-05-23/build.sh#L24 this make any fuzzer with a compiler wrapper like AFL++ and libafl incompatible. These changes don't come from the OSS-Fuzz [build script](https://github.com/google/oss-fuzz/blob/master/projects/mruby/build.sh) as AFL++ works perfectly there.

+ The commit was updated and it is not anymore the version 2018-05-23 (so the name has no sense now) and thus there are no known bugs that can be triggered in a bug-based experiment.