Always read plugin manifest as UTF-8

The manifest can contain UTF-8 data and if no LANG is set (like in
minimal environments), this causes the read to fail. This forces it to
read the file using UTF-8 encoding.

lib/puppet/jenkins/plugins.rb

@@ -54,7 +54,7 @@ def self.available
 
           manifest = File.join(plugin_dir, 'META-INF', 'MANIFEST.MF')
           begin
-            manifest = manifest_data(File.read(manifest))
+            manifest = manifest_data(File.read(manifest, encoding: 'UTF-8'))
             plugins[plugin] = manifest if manifest
           rescue StandardError
             # Nothing really to do about it, failing means no version which will