Seems like since updating to 1.2.0 of hockeyapp, Jenkins has been unable to load the plugin. Here's a excerpt of the log, with the full log attached as a file:

Dec 22, 2014 1:50:45 PM WARNING hudson.ExtensionFinder$Sezpoz scout
Failed to scout net.hockeyapp.jenkins.releaseNotes.ChangelogReleaseNotes$DescriptorImpl
java.lang.InstantiationException: java.lang.UnsupportedClassVersionError: net/hockeyapp/jenkins/releaseNotes/ChangelogReleaseNotes$DescriptorImpl : Unsupported major.minor version 51.0
at net.java.sezpoz.IndexItem.element(IndexItem.java:146)
at hudson.ExtensionFinder$Sezpoz.scout(ExtensionFinder.java:666)
at hudson.ClassicPluginStrategy.findComponents(ClassicPluginStrategy.java:349)
at hudson.ExtensionList.load(ExtensionList.java:300)
at hudson.ExtensionList.ensureLoaded(ExtensionList.java:253)
at hudson.ExtensionList.iterator(ExtensionList.java:143)
at hudson.ClassicPluginStrategy.findComponents(ClassicPluginStrategy.java:348)
at hudson.ExtensionList.load(ExtensionList.java:300)
at hudson.ExtensionList.ensureLoaded(ExtensionList.java:253)
at hudson.ExtensionList.size(ExtensionList.java:162)
at com.sonyericsson.hudson.plugins.gerrit.trigger.PluginImpl.postInitialize(PluginImpl.java:298)
at hudson.PluginManager$2$1$2.run(PluginManager.java:386)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:903)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:695)
Caused by: java.lang.UnsupportedClassVersionError: net/hockeyapp/jenkins/releaseNotes/ChangelogReleaseNotes$DescriptorImpl : Unsupported major.minor version 51.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:637)
at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
at jenkins.util.AntClassLoader.defineClassFromData(AntClassLoader.java:1138)
at hudson.ClassicPluginStrategy$AntClassLoader2.defineClassFromData(ClassicPluginStrategy.java:768)
at jenkins.util.AntClassLoader.getClassFromStream(AntClassLoader.java:1309)
at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1365)
at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1325)
at sun.reflect.GeneratedMethodAccessor3.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at jenkins.ClassLoaderReflectionToolkit.invoke(ClassLoaderReflectionToolkit.java:44)
at jenkins.ClassLoaderReflectionToolkit._findClass(ClassLoaderReflectionToolkit.java:86)
at hudson.PluginManager$UberClassLoader.findClass(PluginManager.java:1110)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at net.java.sezpoz.IndexItem.element(IndexItem.java:134)
... 19 more

Can confirm downgrading to 1.1.0 resolves this issue.