Use HTTPS URLs for RubyGems

This modifies Gemfiles to use HTTPS URLs for RubyGems.

As a minor additional change, solrizer.gemspec now requires RSpec2, because the code

```ruby
  RSpec::Core::RakeTask.new(:rspec) do |spec|
    spec.pattern = FileList['spec/**/*_spec.rb']
  end
```

does not function properly in RSpec3 (running rake yields the error "TypeError: no implicit conversion of Rake::FileList into String").