Use parsing for mail 2.5.x, but `instance_variable_get` for later versions

[spovich]

Add compatibility with mail 2.5 using JSON.parse, but for mail 2.6+, use instance_variable_get.

• Use the appraisal gem so now travis builds against mail 2.5 and 2.6
• cleanup dependencies in gemspec (mail is runtime dependency, not dev dependency)
• make mail minimum version >= 2.5.0
• relax development dependencies except rubocop

[kshnurov]

Closed in favor of monkey-patching mail 2.5 https://github.com/spovich/mandrill_dm/wiki/Rails-3-(Mail-2.5)-support