textproc/ruby-yard: update to 0.9.20

Update ruby-yard to 0.9.20, fixing CVE-2019-14369.

# 0.9.20 - June 27th, 2019

[0.9.20]: https://github.com/lsegal/yard/compare/v0.9.19...v0.9.20

- Fix parsing of stringified Symbols in Ruby source (#1256).
- Fix path traversal vulnerability in `yard server`. This bug would allow
unsanitized HTTP requests to access arbitrary files on the machine of a
`yard server` host under certain conditions. Thanks to CuongMX from
Viettel Cyber Security for discovering this vulnerability.

# 0.9.19 - April 2nd, 2019

[0.9.19]: https://github.com/lsegal/yard/compare/v0.9.16...v0.9.19

- Fixed bug in browser back button (#1071, #1228)
- Fixed handling of ArgumentError in ExtraFileObject (#1198)
- Fixed double return tag displaying on boolean methods (#1226)
- Removed unused `Module#namespace_name` function (#1229)
- Fixed parsing order of README files. YARD will now prefer README over
  README.md over README.x.md or README-x.md (and the like). READMEs will now
  also be ordered by filename; the first README is still chosen unless
  `--readme` is provided.
- Updated AsciiDoc markup support to use non-deprecated calls.

(taca)