gemspec: remove homepage_uri

This raises the following error:

```
WARNING:  You have specified the uri:
  https://github.com/voxpupuli/json-schema/
for all of the following keys:
  homepage_uri
  source_code_uri
Only the first one will be shown on rubygems.org
```

json-schema.gemspec

@@ -7,7 +7,6 @@ Gem::Specification.new do |s|
   s.metadata = {
     'source_code_uri' => s.homepage,
     'changelog_uri' => "#{s.homepage}/blob/master/CHANGELOG.md",
-    'homepage_uri' => s.homepage,
     'bug_tracker_uri' => "#{s.homepage}/issues",
   }
   s.summary = 'Ruby JSON Schema Validator'