yara: 3.7.1 -> 3.8.1

This fixes issues CVE-2018-12034 & CVE-2018-12035. They are OOB read &
write issues of the internal VM. Details can be retrieved at [1] & [2].

[1] https://github.com/VirusTotal/yara/issues/891
[2] https://bnbdr.github.io/posts/swisscheese/