devel/ruby-parser: update to 3.15.0

Update ruby-parser package to 3.15.0.

=== 3.15.0 / 2020-08-31

* 1 major enhancement:

  * Added tentative 2.7 support.

* 1 minor enhancement:

  * Improved ruby_parse_extract_error's handling of moving slow files out.

* 22 bug fixes:

  * Bumped ruby version to include 3.0 (trunk).
  * Fix an error related to empty ensure bodies. (presidentbeef)
  * Fix handling of bad magic encoding comment.
  * Fixed SystemStackError when parsing a huoooge hash, caused by a splat arg.
  * Fixed a number of errors parsing do blocks in strange edge cases.
  * Fixed a string backslash lexing bug when the string is an invalid encoding. (nijikon, gmcgibbon)
  * Fixed bug assigning line number to some arg nodes.
  * Fixed bug concatinating string literals with differing encodings.
  * Fixed bug lexing heredoc w/ nasty mix of \r\n and \n.
  * Fixed bug lexing multiple codepoints in \u{0000 1111 2222} forms.
  * Fixed bug setting line numbers in empty xstrings in some contexts.
  * Fixed edge case on call w/ begin + do block as an arg.
  * Fixed handling of UTF BOM.
  * Fixed handling of lexer state across string interpolation braces.
  * Fixed infinite loop when lexing backslash+cr+newline (aka dos-files)
  * Fixed lambda + do block edge case.
  * Fixed lexing of some ?\M... and ?\C... edge cases.
  * Fixed more do/brace block edge case failures.
  * Fixed parsing bug where splat was used in the middle of a list.
  * Fixed parsing of interpolation in heredoc-like strings. (presidentbeef)
  * Fixed parsing some esoteric edge cases in op_asgn.
  * Fixed unicode processing in ident chars so now they better mix.

(taca)