Does Javassist 3.17.0-GA have to be used with Java 1.7? Is it intentional?

I have the below error. You can reproduce with Java 1.6 and this:

git clone git://github.com/ngocdaothanh/xitrum-new.git

cd xitrum-new

git checkout 1.9.9

sbt/sbt run

java.lang.UnsupportedClassVersionError: javassist/ClassPath : Unsupported major.minor version 51.0

          at java.lang.ClassLoader.defineClass1(Native Method) ~[na:1.6.0_37]

          at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631) ~[na:1.6.0_37]

          at java.lang.ClassLoader.defineClass(ClassLoader.java:615) ~[na:1.6.0_37]

          at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141) ~[na:1.6.0_37]

          at java.net.URLClassLoader.defineClass(URLClassLoader.java:283) ~[na:1.6.0_37]

          at java.net.URLClassLoader.access$000(URLClassLoader.java:58) ~[na:1.6.0_37]

          at java.net.URLClassLoader$1.run(URLClassLoader.java:197) ~[na:1.6.0_37]

          at java.security.AccessController.doPrivileged(Native Method) ~[na:1.6.0_37]

          at java.net.URLClassLoader.findClass(URLClassLoader.java:190) ~[na:1.6.0_37]

          at java.lang.ClassLoader.loadClass(ClassLoader.java:306) ~[na:1.6.0_37]

          at java.lang.ClassLoader.loadClass(ClassLoader.java:247) ~[na:1.6.0_37]

          at xitrum.routing.Routes$.fromCacheFileOrRecollectReal(Routes.scala:195) [xitrum_2.9.2.jar:1.9.10-SNAPSHOT]

          at xitrum.routing.Routes$.fromCacheFileOrRecollectWithRetry(Routes.scala:170) [xitrum_2.9.2.jar:1.9.10-SNAPSHOT]

          at xitrum.routing.Routes$.fromCacheFileOrRecollect(Routes.scala:156) [xitrum_2.9.2.jar:1.9.10-SNAPSHOT]

          at xitrum.handler.Server$.start(Server.scala:18) [xitrum_2.9.2.jar:1.9.10-SNAPSHOT]

          at demos.Boot$.main(Boot.scala:12) [classes/:na]

          at demos.Boot.main(Boot.scala) [classes/:na]

          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.6.0_37]

          at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) ~[na:1.6.0_37]

          at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) ~[na:1.6.0_37]

          at java.lang.reflect.Method.invoke(Method.java:597) ~[na:1.6.0_37]

          at sbt.Run.invokeMain(Run.scala:68) [run-0.12.0.jar:0.12.0]

          at sbt.Run.run0(Run.scala:61) [run-0.12.0.jar:0.12.0]

          at sbt.Run.execute$1(Run.scala:50) [run-0.12.0.jar:0.12.0]

          at sbt.Run$$anonfun$run$1.apply$mcV$sp(Run.scala:54) [run-0.12.0.jar:0.12.0]

          at sbt.TrapExit$.executeMain$1(TrapExit.scala:33) [run-0.12.0.jar:0.12.0]

          at sbt.TrapExit$$anon$1.run(TrapExit.scala:42) [run-0.12.0.jar:0.12.0]