Removed Mutex usage from XML::Text

Instead of trying to make this class thread-safe I'm going with the
option of simply declaring it unsafe to mutate instances of XML::Text
while reading it in parallel. This removes the need for Mutex
allocations and keeps the code simple.

Fixes #82

lib/oga/xml/text.rb

@@ -8,7 +8,6 @@ module Oga
       def initialize(*args)
         super
 
-        @mutex   = Mutex.new
         @decoded = false
       end
 
@@ -16,12 +15,9 @@ module Oga
       # @param [String] value
       #
       def text=(value)
-        # In case of concurrent text/text= calls.
-        @mutex.synchronize do
         @decoded = false
         @text    = value
       end
-      end
 
       ##
       # Returns the text as a String. Upon the first call any XML/HTML entities
@@ -30,13 +26,11 @@ module Oga
       # @return [String]
       #
       def text
-        @mutex.synchronize do
         unless @decoded
           decoder  = html? ? HTML::Entities : Entities
           @text    = decoder.decode(@text)
           @decoded = true
         end
-        end
 
         return @text
       end