Change scala version to 2.10.4 to make it work with Java8 JDK

Hi. I tried your activator template but got the following errors when trying to run it:
```
[info] Compiling 6 Scala sources to /home/bjorn/tmp/spray-actor-per-request/target/scala-2.10/classes...
[info] 'compiler-interface' not yet compiled for Scala 2.10.0. Compiling...
error: error while loading CharSequence, class file '/home/bjorn/devtools/java/jdk1.8.0_05/jre/lib/rt.jar(java/lang/CharSequence.class)' is broken
(class java.lang.RuntimeException/bad constant pool tag 15 at byte 1501)
error: error while loading Comparator, class file '/home/bjorn/devtools/java/jdk1.8.0_05/jre/lib/rt.jar(java/util/Comparator.class)' is broken
(class java.lang.RuntimeException/bad constant pool tag 15 at byte 5003)
error: error while loading AnnotatedElement, class file '/home/bjorn/devtools/java/jdk1.8.0_05/jre/lib/rt.jar(java/lang/reflect/AnnotatedElement.class)' is broken
(class java.lang.RuntimeException/bad constant pool tag 15 at byte 2713)
error: error while loading Arrays, class file '/home/bjorn/devtools/java/jdk1.8.0_05/jre/lib/rt.jar(java/util/Arrays.class)' is broken
(class java.lang.RuntimeException/bad constant pool tag 15 at byte 12801)
/tmp/sbt_462c9f85/xsbt/ExtractAPI.scala:395: error: java.util.Comparator does not take type parameters
        private[this] val sortClasses = new Comparator[Symbol] {
                                            ^
5 errors found
```

This seem to be caused by using a Java8 JDK. Changing to Scala 2.10.4 solved it.