Missing runtime dependency - pry gem

In `lib/xmpp4r/httpbinding/client.rb:12` required gem `pry`. But it doesn't specify in spec dependency.

I don't know how `xmpp4r` use it, so i decide to add it in dependency list, and recreate `.gemspec` file, using `rake gem:update_gemspec`

```ruby
# lib/xmpp4r/httpbinding/client.rb:12

require 'pry'
# ...
```

if this gem doesn't required for xmpp4r you could remove it from `httpbinding/client.rb` and close my request :broken_heart: