Ruby 2+ support

• Replaces ftpfxp (Ruby 1.9 support only) with the double_bag_ftps gem (Ruby 1.9 and 2+ support)
• Drops Ruby 1.9 builds in Travis

Support version 2 of the Retryable gem

• Support Retryable v2.0
• Stop testing Ruby 1.8 with Travis

Change FTP auth timeout default to 30 seconds

• Changed the FTP authentication default timeout 30 seconds
• Some minor refactoring

Upgrade Retryable to version 1.3

• Supporting retryable's :on and :matching options
• Upgraded Retryable to version 1.3
• The Retryable default is now tries: 2, meaning it's retried once (which conceptually preserves how this gem worked previously).
• Added Retryable sleep option, which defaults to 1 second in between tries (which is the retryable gem's default).

Improve README

• Added usage examples to README