Set explicit 3G max heap size for build

While hotspot seems to acquire enough memory to accomodate the heap
 requirements during our builds, the IBM JVM, in the absence of an
 explicit upper limit it caps the heap size to 512M, which is not
 enough for our build. Setting the Xmx value to 3G seems to be
 a good all-around solution.

pom.xml

@@ -32,7 +32,7 @@
     <bouncycastle.version>1.52</bouncycastle.version>
     <generate-config-docs-phase>prepare-package</generate-config-docs-phase>
     <hsqldb.version>2.3.2</hsqldb.version>
-    <test.runner.jvm.settings>-XX:MaxPermSize=128M -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=target/test-data -Dorg.neo4j.unsafe.impl.internal.dragons.UnsafeUtil.DIRTY_MEMORY=true -XX:-OmitStackTraceInFastThrow</test.runner.jvm.settings>
+    <test.runner.jvm.settings>-Xmx3G -XX:MaxPermSize=128M -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=target/test-data -Dorg.neo4j.unsafe.impl.internal.dragons.UnsafeUtil.DIRTY_MEMORY=true -XX:-OmitStackTraceInFastThrow</test.runner.jvm.settings>
     <doclint-groups>reference</doclint-groups>
   </properties>