Update snakeyaml to 1.25 (#2987)

* Update snakeyaml to 1.25

* Fix compile error when loading the build

With the new version, the build fails to load with this error:
```
[info] Compiling 2 Scala sources to /home/travis/build/typelevel/cats/project/target/scala-2.12/sbt-1.0/classes ...
[info] Done compiling.
/home/travis/build/typelevel/cats/build.sbt:43: error: value asScala is not a member of Nothing
      .collect { case map: java.util.Map[_, _] => map.asScala.toList }
                                                      ^
[error] Type error in expression
```

I've no idea why this happens, but this change fixes it. I've verified
that `crossScalaVersions` still contains the versions from `.travis.yml`:

```
sbt:cats> coreJVM/crossScalaVersions
[info] * 2.11.12
[info] * 2.12.9
[info] * 2.13.0
```

build.sbt

@@ -39,11 +39,9 @@ crossScalaVersionsFromTravis in Global := {
   val manifest = (baseDirectory in ThisBuild).value / ".travis.yml"
   import collection.JavaConverters._
   Using.fileInputStream(manifest) { fis =>
-    List(new org.yaml.snakeyaml.Yaml().load(fis))
-      .collect { case map: java.util.Map[_, _] => map.asScala.toList }
-      .flatMap(_.collect {
-        case (k: String, v: String) if k.contains("scala_version_") => v
-      })
+    new org.yaml.snakeyaml.Yaml().loadAs(fis, classOf[java.util.Map[_, _]]).asScala.toList.collect {
+      case (k: String, v: String) if k.contains("scala_version_") => v
+    }
   }
 }
 

project/build.sbt

@@ -1 +1 @@
-libraryDependencies += "org.yaml" % "snakeyaml" % "1.24"
+libraryDependencies += "org.yaml" % "snakeyaml" % "1.25"