Fix deprecation warnings about using URI.escape method

[Envek]

• [x] Replace obsolete URI.escape method with CGI.escape (no specs failed so it should be fine, I think)
• [x] Fix deprecation warnings about using ActionView::Base.new in specs
• [x] Include newer Rails and Ruby versions into CI matrix

[olimart]

@mdeering would be great to get this one merged. Thanks

[szechyjs]

@mdeering any chance of getting this merged and released?

[andyjeffries]

If we can't get it merged and @mdeering isn't responding, is there a more up to date fork of this gem the community is using?

[olimart]

Get rid of the dependency with a simple helper

hash = Digest::MD5.hexdigest(email)
"https://secure.gravatar.com/avatar/#{hash}.png?height=#{size}&width=#{size}"

[michaelglass]

thanks @olimart! the url above should be

"https://secure.gravatar.com/avatar/#{hash}.png?s=#{size}"

see: https://en.gravatar.com/site/implement/images#size

[paulhklam1122]

@mdeering can we get this merged, please? It breaks when upgrading to ruby 3. Thank you!