Fix to make this compile/valid in Ruby 2.0

Enforce "us-ascii" encoding in the lib/oai/client file so the
invalid utf-8 characters don't cause a syntax error.

Fixes #30

Author: dazoakley

lib/oai/client.rb

@@ -1,3 +1,5 @@
+# encoding: us-ascii
+
 # External dependencies
 require 'uri'
 require 'faraday'