Fix uninitialized constant

`<class:Client>': uninitialized constant Forwardable (NameError)
0.5.1
Use HTTPS_PROXY environment variable

The Trello API uses the https protocol, so the gem should
use the https proxy environment variable accordingly.
Restrict rescue statement to RestClient::Exception

Exception#http_code and Exception#http_body are only provided by
RestClient::Exception, so restrict the rescue statement to those
types of exceptions.
reverts #48