Prefer thread-safe mode.

https://ruby-doc.org/stdlib-2.7.0/libdoc/pstore/rdoc/PStore.html#method-c-new

Author: mostlyobvious

lib/nanoc/github.rb

@@ -10,7 +10,7 @@ module Github
 
     class Cache
       def initialize(cache_dir)
-        @store = PStore.new(File.join(cache_dir, "nanoc-github.store"))
+        @store = PStore.new(File.join(cache_dir, "nanoc-github.store"), true)
       end
 
       def write(name, value, options = nil)