fix failure without preJDK7 flag

Hi there,
I was playing with scalameter and found that master is kind of broken.
When I tried to run a Quickbenchmark it gave me  the following exception

Exception in thread "main" java.util.NoSuchElementException: key not found: pre-jdk-7
	at scala.collection.MapLike$class.default(MapLike.scala:228)
	at scala.collection.AbstractMap.default(Map.scala:58)
	at scala.collection.MapLike$class.apply(MapLike.scala:141)
	at scala.collection.AbstractMap.apply(Map.scala:58)
	at org.scalameter.Context.apply(package.scala:175)
	at org.scalameter.utils.package$$anon$2.apply(package.scala:14)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at org.scalameter.Executor$Warmer$Default$$anon$1.foreach(Executor.scala:62)
	at org.scalameter.execution.LocalExecutor$$anonfun$run$1$$anonfun$apply$1.apply(LocalExecutor.scala:40)
	at org.scalameter.execution.LocalExecutor$$anonfun$run$1$$anonfun$apply$1.apply(LocalExecutor.scala:37)
	at scala.collection.Iterator$class.foreach(Iterator.scala:727)
	at scala.collection.AbstractIterator.foreach(Iterator.scala:1156)
	at org.scalameter.execution.LocalExecutor$$anonfun$run$1.apply(LocalExecutor.scala:37)
	at org.scalameter.execution.LocalExecutor$$anonfun$run$1.apply(LocalExecutor.scala:35)
	at org.scalameter.utils.Tree$$anonfun$foreach$2$$anonfun$apply$1.apply(Tree.scala:13)
	at org.scalameter.utils.Tree$$anonfun$foreach$1.apply(Tree.scala:12)
	at scala.collection.immutable.List.foreach(List.scala:309)
	at org.scalameter.utils.Tree.foreach(Tree.scala:12)
	at org.scalameter.utils.Tree$$anonfun$foreach$2.apply(Tree.scala:13)
	at org.scalameter.utils.Tree$$anonfun$foreach$2.apply(Tree.scala:13)
	at scala.collection.immutable.List.foreach(List.scala:309)
	at org.scalameter.utils.Tree.foreach(Tree.scala:13)
	at org.scalameter.execution.LocalExecutor.run(LocalExecutor.scala:35)
	at org.scalameter.PerformanceTest$Initialization$class.executeTests(PerformanceTest.scala:43)
	at org.scalameter.PerformanceTest.executeTests(PerformanceTest.scala:10)
	at org.scalameter.PerformanceTest$$anonfun$1.apply$mcZ$sp(PerformanceTest.scala:15)
	at org.scalameter.PerformanceTest$$anonfun$1.apply(PerformanceTest.scala:15)
	at org.scalameter.PerformanceTest$$anonfun$1.apply(PerformanceTest.scala:15)
	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:57)
	at org.scalameter.PerformanceTest.main(PerformanceTest.scala:14)
	at com.jozic.sorts.SortBenchmark.main(SortBenchmark.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

So I've fixed it and have added some simple tests.
I would be glad to hear your code review comments and fix them, if needed.

Thanks,
Eugene