[ANN] sexp_processor 4.15.1 Released

sexp_processor version 4.15.1 has been released!

* home: <https://github.com/seattlerb/sexp_processor>
* rdoc: <http://docs.seattlerb.org/sexp_processor>

sexp_processor branches from ParseTree bringing all the generic sexp
processing tools with it. Sexp, SexpProcessor, Environment, etc... all
for your language processing pleasure.

Changes:

### 4.15.1 / 2020-08-31

* 1 bug fix:

  * Bumped ruby version to include 3.0 (trunk).