Fix regular expression for ruby 1.8 compatibility

[emyl]

Without escaping curly braces all commands fail with the following error on ruby 1.8:

/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require': /var/lib/gems/1.8/gems/guides-0.7.2/lib/guides/indexer.rb:49: invalid regular expression; there's no previous pattern, to which '{' would define cardinality at 1: /{{(.*?)}}/ (SyntaxError)
        from /usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `require'
        from /var/lib/gems/1.8/gems/guides-0.7.2/lib/guides/generator.rb:56
        from /usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
        from /usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `require'
        from /var/lib/gems/1.8/gems/guides-0.7.2/lib/guides.rb:7
        from /usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
        from /usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `require'
        from /var/lib/gems/1.8/gems/guides-0.7.2/bin/guides:3
        from /var/lib/gems/1.8/bin/guides:19:in `load'
        from /var/lib/gems/1.8/bin/guides:19