Use HTTPS to connect to crowdin.com

When doing

````ruby
crowdin = Crowdin::API.new project_id: 'openproject', api_key: '[secret]'
crowdin.project_info
````

I get an error:

    crowdin.project_info
    JSON::ParserError: 795: unexpected token at '<html>
    <head><title>301 Moved Permanently</title></head>
    <body bgcolor="white">
    <center><h1>301 Moved Permanently</h1></center>
    <hr><center>nginx</center>
    </body>
    </html>
    '
    from /home/tessi/.rbenv/versions/2.1.1/lib/ruby/2.1.0/json/common.rb:155:in `parse'

Changing the `base_url` to HTTPS works (and is more secure \o/)