Fix hostname mismatch caused by new gem

Pin to travis-artifacts 0.2.2 to fix hostname mismatch errors with 0.3.0

The new version of travis-artifacts is preventing our builds from
uploading to S3 with the following error:

/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)

And the overall build did not fail because the upload tasks are part of the
after_success stage. This was intentional, but we may want to consider a
build success to include the whole pipeline...