Tar flags

This change makes the flags in the untar command conditional on
whether the binary is actually gzipped.  On Linux (unlike OS X), using
the z flag causes a failure if the file isn't actually gzipped.

Author: nebhale

lib/java_buildpack/component/base_component.rb

@@ -121,7 +121,7 @@ def download_tar(version, uri, target_directory = @droplet.sandbox, name = @comp
         download(version, uri, name) do |file|
           with_timing "Expanding #{name} to #{target_directory.relative_path_from(@droplet.root)}" do
             FileUtils.mkdir_p target_directory
-            shell "tar xzf #{file.path} -C #{target_directory} --strip 1 2>&1"
+            shell "tar x#{gzipped?(file) ? 'x' : ''}f #{file.path} -C #{target_directory} --strip 1 2>&1"
           end
         end
       end
@@ -165,6 +165,12 @@ def with_timing(caption)
         puts "(#{(Time.now - start_time).duration})"
       end
 
+      private
+
+      def gzipped?(file)
+        file.path.end_with? '.gz'
+      end
+
     end
 
   end