Support Configuration as code plugin

This plugin doesn't seem to support casc.

Config json schema of this plugin doesn't look right

also, configuration variables can't be set.
```plain
2019-11-14 09:23:38.965+0000 [id=19] SEVERE hudson.util.BootFailure#publish: Failed to initialize Jenkins
io.jenkins.plugins.casc.ConfiguratorException: Invalid configuration elements for type class com.github.terma.jenkins.githubprcoveragestatus.Configuration$ConfigurationDescriptor : personalAccessToken,jenkinsUrl.
Available attributes :
at io.jenkins.plugins.casc.BaseConfigurator.handleUnknown(BaseConfigurator.java:377)
at io.jenkins.plugins.casc.BaseConfigurator.configure(BaseConfigurator.java:366)
at io.jenkins.plugins.casc.BaseConfigurator.check(BaseConfigurator.java:287)
at io.jenkins.plugins.casc.BaseConfigurator.configure(BaseConfigurator.java:352)
at io.jenkins.plugins.casc.BaseConfigurator.check(BaseConfigurator.java:287)
at io.jenkins.plugins.casc.ConfigurationAsCode.lambda$checkWith$7(ConfigurationAsCode.java:716)
at io.jenkins.plugins.casc.ConfigurationAsCode.invokeWith(ConfigurationAsCode.java:666)
Caused: io.jenkins.plugins.casc.ConfiguratorException: unclassified: error configuring 'unclassified' with class io.jenkins.plugins.casc.impl.configurators.GlobalConfigurationCategoryConfigurator configurator
at io.jenkins.plugins.casc.ConfigurationAsCode.invokeWith(ConfigurationAsCode.java:672)
at io.jenkins.plugins.casc.ConfigurationAsCode.checkWith(ConfigurationAsCode.java:716)
at io.jenkins.plugins.casc.ConfigurationAsCode.configureWith(ConfigurationAsCode.java:701)
at io.jenkins.plugins.casc.ConfigurationAsCode.configureWith(ConfigurationAsCode.java:592)
at io.jenkins.plugins.casc.ConfigurationAsCode.configure(ConfigurationAsCode.java:281)
at io.jenkins.plugins.casc.ConfigurationAsCode.init(ConfigurationAsCode.java:273)
Caused: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at hudson.init.TaskMethodFinder.invoke(TaskMethodFinder.java:104)
Caused: java.lang.Error
at hudson.init.TaskMethodFinder.invoke(TaskMethodFinder.java:110)
at hudson.init.TaskMethodFinder$TaskImpl.run(TaskMethodFinder.java:175)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:296)
at jenkins.model.Jenkins$5.runTask(Jenkins.java:1118)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:214)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused: org.jvnet.hudson.reactor.ReactorException
at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:282)
at jenkins.InitReactorRunner.run(InitReactorRunner.java:48)
at jenkins.model.Jenkins.executeReactor(Jenkins.java:1152)
at jenkins.model.Jenkins.(Jenkins.java:959)
at hudson.model.Hudson.(Hudson.java:85)
at hudson.model.Hudson.(Hudson.java:81)
at hudson.WebAppMain$3.run(WebAppMain.java:233)
Caused: hudson.util.HudsonFailedToLoad
at hudson.WebAppMain$3.run(WebAppMain.java:250)
```