DEPS: Update jsoncpp to 1.9.3 (#6308)

Updates jsoncpp to 1.9.3. Doesn't look like any of the tests broke.

* [x] I work here
* [x] Ran existing tests

I'll file a follow-up PR for adding trailing comma support, as mentioned in #4232 (comment)

dep/jsoncpp/README.md

@@ -2,6 +2,15 @@
 
 [Amalgamated](https://github.com/open-source-parsers/jsoncpp/wiki/Amalgamated)
 from source commit
-[ddabf50](https://github.com/open-source-parsers/jsoncpp/commit/ddabf50f72cf369bf652a95c4d9fe31a1865a781),
-release 1.8.4.
+[6aba23f](https://github.com/open-source-parsers/jsoncpp/commit/6aba23f4a8628d599a9ef7fa4811c4ff6e4070e2),
+release 1.9.3.
 
+> Generating amalgamated source and header JsonCpp is provided with a script to
+> generate a single header and a single source file to ease inclusion into an
+> existing project. The amalgamated source can be generated at any time by
+> running the following command from the top-directory (this requires Python
+> 3.4+):
+>
+> ```
+> python amalgamate.py
+> ```

dep/jsoncpp/cgmanifest.json

@@ -0,0 +1,13 @@
+{"Registrations":[
+    {
+      "component": {
+       "type": "git",
+       "git": {
+         "repositoryUrl": "https://github.com/open-source-parsers/jsoncpp",
+         "commitHash": "6aba23f4a8628d599a9ef7fa4811c4ff6e4070e2"
+         }
+       }
+    }
+ ],
+ "Version": 1
+}