CHANGE to use changelog uri from rubygems metadata

[MaximeD]

⚠️ This is a rewrite of how gem_updater works.

We used to try to guess where the changelog was located by opening the repository’s page, and scrap it to find where the changelog is located. This had its downsides, mainly we were quite dependant of the markup of the page, which could change if github decided to, also this was just a guess as the changelog could have been named CHANGELOG.md, NEWS.txt, changes, or whatever else.

Instead we will now rely on the gem’s metadata, which MAY include a property changelog_uri. If this property is there, then this is where our changelog is, and the only ice on the cake is that we look for an anchor that matches the version we updated to. If the property is not there, this is a good opportunity for an easy contribution on the gem’s repository to add where its changelog is located, but that won’t be the responsibility of gem_updater to try to find it.

[MaximeD]

Addresses #44 , #50 and #166