Provide a 'Changelog' link on rubygems.org/gems/spreadsheet

[mark-young-atg]

By providing a 'changelog_uri' in the metadata of the gemspec a 'Changelog' link will be shown on https://rubygems.org/gems/spreadsheet which makes it quick and easy for someone to check on the changes introduced with a new version.

Details of this functionality can be found on https://guides.rubygems.org/specification-reference/

[zdavatz]

Thank you for the PR! Can you please make sure that all the tests pass, then I will merge an release a new version!

[mark-young-atg]

@zdavatz Thank you for your review.
I can confirm that the tests pass successfully locally

$ test/suite.rb
/Users/user/src/spreadsheet/test/integration.rb:1405: warning: assigned but unused variable - path
Loaded suite test/suite
Started
Finished in 1.688957 seconds.
---------------------------------------------------------------------------------------------------------------------------------------------------------------
93 tests, 1142 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
---------------------------------------------------------------------------------------------------------------------------------------------------------------
55.06 tests/s, 676.16 assertions/s
Coverage report generated for Unit Tests to /Users/user/src/spreadsheet/coverage. 4102 / 4349 LOC (94.32%) covered.

The change I made in this PR is to a metadata hash value in the gemspec file. I don't see any correlation between that change and the errors in the full test suite. Can you confirm that the test suite runs successfully against the master branch? Also the 'test (ubuntu, 2,2)' test seems to have failed to run, rather than failed due to a coding error.

Also note that the test failures for my other PR failed with the same errors. That suggests to me that the test failures are not due to the changes I'm making.