Try to find the ffmpeg binary automatically

lib/streamio-ffmpeg.rb

@@ -39,10 +39,27 @@ def self.ffmpeg_binary=(bin)
     @ffmpeg_binary = bin
   end
 
-  # Get the path to the ffmpeg binary, defaulting to 'ffmpeg'
+  # Get the path to the ffmpeg binary
   #
   # @return [String] the path to the ffmpeg binary
   def self.ffmpeg_binary
-    @ffmpeg_binary || 'ffmpeg'
+    @ffmpeg_binary ||= find_ffmpeg_binary
   end
+
+  # Tries find the ffmpeg binary
+  #
+  # @return [String] the path to the ffmpeg binary
+  def self.find_ffmpeg_binary
+    %w(
+      /usr/bin/avconv
+      /usr/bin/ffmpeg
+      /usr/local/bin/avconv
+      /usr/local/bin/ffmpeg
+    ).each do |path|
+      return path if File.exists?(path)
+    end
+
+    raise "unable to find ffmpeg binary"
+  end
+
 end