Soften Nokogiri dependency to versions ">= 1.4"

Many Ruby / Rails projects already use Nokogiri 1.6.6.x series.
<= 1.6.5 make these projects impossible to integrate html-pipeline.

Use real XPath with search() in emoji_filter and mention_filter
Use #search with a real XPath expression to avoid the need to convert CSS
into a hacky XPath expression. This workaround mentioned in http://git.io/vmp2F

html-pipeline.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |gem|
   gem.test_files    = gem.files.grep(%r{^test})
   gem.require_paths = ["lib"]
 
-  gem.add_dependency "nokogiri", [">= 1.4", "<= 1.6.5"]
+  gem.add_dependency "nokogiri", ">= 1.4"
   gem.add_dependency "activesupport", [">= 2", "< 5"]
 
   gem.post_install_message = <<msg

lib/html/pipeline/@mention_filter.rb

@@ -70,7 +70,7 @@ def self.mentioned_logins_in(text, username_pattern=UsernamePattern)
       def call
         result[:mentioned_usernames] ||= []
 
-        doc.search('text()').each do |node|
+        doc.search('.//text()').each do |node|
           content = node.to_html
           next if !content.include?('@')
           next if has_ancestor?(node, IGNORE_PARENTS)

lib/html/pipeline/emoji_filter.rb

@@ -19,7 +19,7 @@ class EmojiFilter < Filter
       DEFAULT_IGNORED_ANCESTOR_TAGS = %w(pre code tt).freeze
 
       def call
-        doc.search('text()').each do |node|
+        doc.search('.//text()').each do |node|
           content = node.to_html
           next unless content.include?(':')
           next if has_ancestor?(node, ignored_ancestor_tags)