net-mgmt/riemann: update to 0.3.8

While Riemann is not expected to be exposed to the Log4J vulnerability,
this release includes updated dependencies as a preventative measure.

https://github.com/riemann/riemann/compare/0.3.7...0.3.8

https://unit42.paloaltonetworks.com/apache-log4j-vulnerability-cve-2021-44228/

We do not believe Riemann is vulnerable but we’ve updated:

• slf4j-over-log4j to 1.7.32 (although we use Logback internally - which is not vulnerable to the log4j issue)
• clojure/tools.logging to 1.2.1 (which pulls in a later and secure version of log4j in as a dependency)

Security: CVE-2021-44228