Pin pstore dependency to test with ruby/pstore#67

Author: nobu

Gemfile

@@ -1,5 +1,8 @@
 source "https://rubygems.org"
 
+gem "pstore", git: "https://github.com/nobu/pstore.git",
+              ref: "1d7b517a700addb0839c3472419203e764243160"
+
 group :development do
   gem "bundler"
   gem "rake"