power_assert repository is transferred to ruby/power_assert

gems/bundled_gems

@@ -1,5 +1,5 @@
 minitest 5.14.0 https://github.com/seattlerb/minitest
-power_assert 1.1.7 https://github.com/k-tsj/power_assert
+power_assert 1.2.0 https://github.com/ruby/power_assert
 rake 13.0.1 https://github.com/ruby/rake
 test-unit 3.3.5 https://github.com/test-unit/test-unit
 rexml 3.2.4 https://github.com/ruby/rexml