Update SnakeYAML to 1.29 (was 1.25: vulnerable to "billion laughs")

Kudos to Noriko Totsuka of @JPCERTCC (and also the author of the
vulnerability report, Taichi Kotake of Akatsuki Inc.) for reporting the
"billion laughs" vulnerability in SnakeYAML 1.25 (that the JVM build of
Kaitai Struct compiler has been using so far) to us.

Note: I am deliberately not updating to the latest version 1.30 because
it contains a suspicious regular expression, see
https://bitbucket.org/snakeyaml/snakeyaml/issues/537/potentially-problematic-regular-expression.
At least until someone proves it's safe under all circumstances, or
fixes it to a 100% harmless version as I suggest, I won't be using any
newer version than 1.29 unless I know it's safe.

build.sbt

@@ -58,7 +58,7 @@ lazy val compiler = crossProject.in(file(".")).
     libraryDependencies ++= Seq(
       "com.github.scopt" %%% "scopt" % "3.6.0",
       "com.lihaoyi" %%% "fastparse" % "1.0.0",
-      "org.yaml" % "snakeyaml" % "1.25"
+      "org.yaml" % "snakeyaml" % "1.29"
     )
   ).
   jvmSettings(