Updated to Ruby 2.6.5.

Resolves several security vulnerabilities:

- CVE-2019-16255: A code injection vulnerability of Shell#[] and
  Shell#test
- CVE-2019-16254: HTTP response splitting in WEBrick (Additional fix)
- CVE-2019-15845: A NUL injection vulnerability of File.fnmatch and
  File.fnmatch?
- CVE-2019-16201: Regular Expression Denial of Service vulnerability of
  WEBrick’s Digest access authentication

[Details](https://is.gd/qWSrzR)

.ruby-version

@@ -1 +1 @@
-2.6.4
+2.6.5