Update excludes for ostruct

The two existing failures were resolved:

* test_method_missing depends on the native method_missing not
  appearing in NoMethodError backtrace.
* test_to_h_with_block was likely fixed by updating ostruct.

Two new failures are due to ruby/ostruct#41 and will be fixed by
ruby/ostruct#45: test_each_pair, test_initialize

test/mri/excludes/TC_OpenStruct.rb

@@ -1,2 +1,2 @@
-exclude :test_method_missing, "depends on __callee__ which is broken"
-exclude :test_to_h_with_block, "test is newer than the released ostruct 0.1.0 (https://github.com/ruby/ostruct/issues/31)"
+exclude :test_each_pair, "Bug in each_pair in ostruct 0.5.4 (https://github.com/ruby/ostruct/pull/45)"
+exclude :test_initialize, "Bug in each_pair in ostruct 0.5.4 (https://github.com/ruby/ostruct/pull/45)"