Retire support for Ruby 1.8.x (MRI) and Ruby Enterprise Edition (REE) based on version 1.8.x.

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>
Fix. It cannot be a constant as it will be loaded multiple times and hence re-defined.

This is to fix the following:

  (eval):23: warning: already initialized constant SIGNING_KEY
  /home/travis/build/kwilczynski/ruby-fizzbuzz/ruby-fizzbuzz.gemspec:23: warning: previous definition of SIGNING_KEY was here

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>