Test pinning to travis-artifacts 0.2.3

Trying to resolve the issue that started happening after May 4th
(artifacts stopped uploading) and interestingly the travis-artifacts
version was bumped to 0.3.0 on May 5th.

Error we are seeing:
/home/travis/.rvm/rubies/ruby-1.9.3-p545/lib/ruby/1.9.1/openssl/ssl-internal.rb:129:in
`post_connection_check': hostname does not match the server certificate
(OpenSSL::SSL::SSLError) (Excon::Errors::SocketError)