Silence Downloader's output

Hello,

This is just a tiny pull request that silences the output of the `Downloader` class when running the `multibyte_conformance_test.rb` file. This output isn't used anywhere for assertions so we can simply remove it. The introducing commit was f238d495. Before:

~~~
$ rm /tmp/cache/NormalizationTest.txt &>> /dev/null

$ ruby -Itest test/multibyte_conformance_test.rb
Run options: --seed 39629

# Running:

Downloading http://www.unicode.org/Public/6.3.0/ucd/NormalizationTest.txt to /tmp/cache/NormalizationTest.txt
....

Finished in 16.974811s, 0.2356 runs/s, 21652.0826 assertions/s.

4 runs, 367540 assertions, 0 failures, 0 errors, 0 skips
~~~

After:

~~~
$ rm /tmp/cache/NormalizationTest.txt &>> /dev/null

$ ruby -Itest test/multibyte_conformance_test.rb
Run options: --seed 48022

# Running:

....

Finished in 16.288564s, 0.2456 runs/s, 22564.2978 assertions/s.

4 runs, 367540 assertions, 0 failures, 0 errors, 0 skips
~~~

Have a nice day.