Omit StringIO#each specs that check VERSION

The JRuby version of StringIO does not define VERSION, but since
this version check happens within the setup for the specs it
cannot be tagged easily.

This commit omits the StringIO specs from our CI runs altogether
until we can address the missing constant.

See ruby/stringio#57

spec/jruby.mspec

@@ -83,6 +83,9 @@ class MSpecScript
     get(:core) << '^' + SPEC_DIR + '/core/process'
   end
 
+  # exclude stringio specs until we can address ruby/stringio#57
+  get(:library) << '^' + SPEC_DIR + '/library/stringio/each'
+
   # prepare exclusion tags
   set(:xtags, get(:xtags) || [])
   set(:ci_xtags, get(:ci_xtags) || [])