Use Swift metadata 'last-mod-timestamp' instead of 'timestamp'

[jefferya]

Fixes #14

• Fixes test to determine if the OpenStack Swift object is outdated with respect to the CWRC repository objects (use 'last-mod-timestamp' metadata key).
• updates bundler version
• adds Ruby 2.4 to the Travis tests

A request for the metadata from the OpenStack Swift instance returns a key last-mod-timestamp

irb(main):056:0> p swift_file.metadata
{"project-id"=>"tpatt_f39b0d8f-ffb4-4e31-86d5-f8fbb90ac38c", "last-mod-timestamp"=>"2019-01-15T04:56:33.772Z", "aip-version"=>"1.0", "project"=>"CWRC", "promise"=>"bronze"}
=> {"project-id"=>"tpatt_f39b0d8f-ffb4-4e31-86d5-f8fbb90ac38c", "last-mod-timestamp"=>"2019-01-15T04:56:33.772Z", "aip-version"=>"1.0", "project"=>"CWRC", "promise"=>"bronze"}