Add ssl_version option to choose SSL version to use

[htanata]

This allows us to do these things:

# Per class option:
class Foo
  include HTTParty

  ssl_version :SSLv3
end

# Override default option:
Foo.get '/list', :ssl_version => :SSLv23

# Per request option:
HTTParty.get 'https://foobar.com/list', :ssl_version => :TLSv1

[travisbot]

This pull request fails (merged c2cf65f into 9c08f47).

[travisbot]

This pull request fails (merged 628f29f1 into 9c08f47).

[travisbot]

This pull request passes (merged 6b88a95 into 9c08f47).

[maxstoller]

👍

[maxstoller]

Bump 😁