Use ssl_version as string on httpclient adapter

Fix issue #86 

On others adapters, we don't have to change.

* **EM-HTTP-Request** doesn't support SSL.
* **Net/HTTP** seems [happy with symbols](http://www.ruby-doc.org/stdlib-1.9.3/libdoc/openssl/rdoc/OpenSSL/SSL/SSLContext.html#method-i-ssl_version-3D)
* **Curb** doesn't have this problem, because [use numbers](https://github.com/savonrb/httpi/blob/master/lib/httpi/adapter/curb.rb#L94).

By now, i'm oficially giving up to find a solution to run jruby with SSL support.