Make ruby-ntlm an optional dependency

[tjarratt]

Decided to bump the minor version number because this is technically a breaking change in the API.

Fixes #107

[rogerleite]

I'm thinking in one scenario. If user doesn't have rubyntlm and do a request with "ntlm" auth, i'm not sure what error message will appears to him.

@tjarratt do you think is too much check if is NTLM is loaded on line:

negotiate_ntlm_auth(http, &requester) if @request.auth.ntlm? # https://github.com/savonrb/httpi/pull/108/files#diff-f34ff4ddc1edbabe503808c45b11cb51L67?

?

[tjarratt]

@rogerleite That's a great idea. Currently they would see a debug message when net_http.rb is required, but I think improving the error message around the code you pointed out is a good idea.

[tjarratt]

Added a fatal log and a small test for this behavior. I felt bad about submitting a PR initially that did not have any tests.

[rogerleite]

Great:exclamation: :shipit:

BTW, looking at Travis, version 1.8.7 had another problem with dependencies. We should drop this version.

[tjarratt]

👍