Install default Java version (dotnet#14)

We should install the default Java version for a given distro, rather than specifically installing `openjdk-7` (which isn't available on Ubuntu 16.04).

BuildAgent/SetupScripts/BuildAgentSetup_Ubuntu.sh

@@ -42,7 +42,7 @@ echo "Installing libunwind8..."
 apt-get install -y libunwind8
 
 echo "Installing Java..."
-apt-get install -y openjdk-7-jre-headless unzip
+apt-get install -y default-jre-headless unzip
 
 echo "Installing Node.js..."
 apt-get install -y nodejs