Make lumberjack compatible with stdlib logger

The Logger from the ruby standard library will use the program name as
the message if both a message and block are missing.

From the docs
  `progname`
    Program name string. Can be omitted. Treated as a message if no
    `message` and `block` are given.
compatibility with Ruby 2.1.1 when Pathname is passed in the Logger initializer; update specs to remove deprecated methods
Merge branch 'compatibility_fix' of https://github.com/devin-c/lumberjack into devin-c-compatibility_fix
remove deprecated rspec method