Fixing compatibility with linux os

Thank you, Travis! Turns out the cairo gem isn't perfectly the same in Linux...

lib/squib/card.rb

@@ -25,7 +25,7 @@ module Squib
       @svgfile       = "#{deck.dir}/#{deck.prefix}#{deck.count_format % index}.svg"
       @cairo_surface = make_surface(@svgfile, backend)
       @cairo_context = Squib::Graphics::CairoContextWrapper.new(Cairo::Context.new(@cairo_surface))
-      @cairo_context.antialias = @deck.antialias
+      @cairo_context.antialias = ANTIALIAS_OPTS[(@deck.antialias.downcase)] || 'subpixel'
     end
 
     def make_surface(svgfile, backend)

lib/squib/constants.rb

@@ -74,6 +74,12 @@ module Squib
       'progress_bar'  => false,
     }
 
+    #Translate the hints to the methods.
+    ANTIALIAS_OPTS = {
+      'best' => 'subpixel',
+      'good' => 'gray',
+      'fast' => 'gray',
+    }
     # These are parameters that are intended to be "expanded" across
     # range if they are singletons.
     #