Replace JacksonFactory with GsonFactory.

[Capstan]

Fixes #210.

[Capstan]

I cannot get the gradle wrapper to work as is:

$ ./gradlew build test
FAILURE: Build failed with an exception.

* What went wrong:
Failed to load native library 'libnative-platform.so' for Linux amd64.

The installed version of gradle I have (4.4.1) runs into other errors.

[Capstan]

Apparently this is at least gradle/gradle#1493, because after 3.5 I get a different error, and it appears because I have the JDK11 installed, it confuses anything earlier than Gradle 5.0 :/

[Capstan]

/cc @josheinhorn