UnsupportedClassVersionError

Running the emulator after `firebase setup:emulators:firestore` gives the following error:

```
firebase serve --only firestore
✔ firestore: started on http://localhost:8080
⚠ firestore: Exception in thread "main"
⚠ firestore: java.lang.UnsupportedClassVersionError: com/google/cloud/datastore/emulator/firestore/CloudFirestore : Unsupported major.minor version 52.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
⚠ firestore:
at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
⚠ firestore:
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:482)
⚠ firestore:

Error: firestore: emulator has exited with code: 1
```