Upgrade rubygems in CI

We want to use rubygems versions that keep requirements as originally
specified instead of sorting them, since it's the behavior of the most
recent rubygems. So we revert commit
5efae43 that first changed the
expectation to "sorted requirements".

Because of the revert, we need to make sure we test against rubygems
versions that don't sort requirements. That means either versions higher
than 3.1.0.pre1, because they include [the change where sorting was
reverted](rubygems/rubygems@f2d9517),
or versions lower than 2.7.8, because they don't include [the change
where the sorting was first
introduced](rubygems/rubygems@8ce0598).

So, since the latest rubygems doesn't support old rubies, install 2.7.7
in the case of ruby 2.0.0, and the latest rubygems otherwise.

.travis.yml

@@ -27,7 +27,12 @@ env:
   - LANG="en_US.UTF-8"
 
 install:
-  - gem update --system 2.7.10
+  - if [ "$TRAVIS_RUBY_VERSION" = "2.0.0" ]; then
+      gem update --system 2.7.7;
+    else
+      gem update --system 3.1.3;
+    fi
+
   - gem install bundler -v "~> 1.17"
   - rake bootstrap
 

spec/resolver_spec.rb

@@ -182,7 +182,7 @@ def self.save!(path, name, index, requirements, resolved)
 Unable to satisfy the following requirements:
 
 - `json (>= 1.7.7)` required by `berkshelf (2.0.7)`
-- `json (>= 1.4.4, <= 1.7.7)` required by `chef (10.26)`
+- `json (<= 1.7.7, >= 1.4.4)` required by `chef (10.26)`
           EOS
           expect(conflict.message_with_trees(:version_for_spec => lambda(&:version))).to eq <<-EOS.strip
 Molinillo could not find compatible versions for possibility named "json":
@@ -193,7 +193,7 @@ def self.save!(path, name, index, requirements, resolved)
 
     chef_app_error (>= 0) was resolved to 1.0.0, which depends on
       chef (~> 10.26) was resolved to 10.26, which depends on
-        json (>= 1.4.4, <= 1.7.7)
+        json (<= 1.7.7, >= 1.4.4)
           EOS
         end
       end