Building mruby is done with `make clean all -j$(nproc)`, but build fails
randomly because `clean` and `all` run in parallel, and `clean` may be
executed after `all` starts.
Therefore, change it as follows.
* Remove `clean` because it seems to be unneeded in the first place.
* Use `rake` command because mruby uses Rake for builds (`Makefile` is a
limited wrapper for calling `rake`).
* Use `-m` option to run builds in parallel.
* Remove `-j` option because build time seems to be almost the same even
if `-j` option is specified.