metadata: fix failure to get exif data for compressed SWF files.

Fix Exiftool not being able to get the metadata for compressed SWF
files. Exiftool requires Compress::Zlib as an optional dependency to
decompress compressed SWF files, but it wasn't in the Docker image.

Archive::Zip is required for Zip files and Digest::MD5 for certain other
metadata (see "DEPENDENCIES" in exiftool README).

test/unit/media_file_test.rb

@@ -180,6 +180,16 @@ class MediaFileTest < ActiveSupport::TestCase
     end
   end
 
+  context "a compressed SWF file" do
+    should "get all the metadata" do
+      @metadata = MediaFile.open("test/files/compressed.swf").metadata
+
+      assert_equal(true, @metadata["Flash:Compressed"])
+      assert_not_equal("Install Compress::Zlib to extract compressed information", @metadata["ExifTool:Warning"])
+      assert_equal(6, @metadata.count)
+    end
+  end
+
   context "a greyscale image without an embedded color profile" do
     should "successfully generate a thumbnail" do
       @image = MediaFile.open("test/files/test-grey-no-profile.jpg")