Suppress Ruby interpreter warnings

[koic]

This PR suppresses the following Ruby interpreter warnings.

% ruby -v
ruby 2.6.0dev (2018-08-28 trunk 64584) [x86_64-darwin17]
% RUBYOPT='-w' rake
/Users/koic/src/github.com/mojombo/chronic/lib/chronic/tags/scalar.rb:32: warning: mismatched indentat
ions at 'end' with 'unless' at 25
Run options: --seed 2694

# Running:

................................................/Users/koic/src/github.com/mojombo/chronic/lib/chronic/repeaters/repeater_quarter.rb:22: warning: instance variable @current_span not initialized
/Users/koic/src/github.com/mojombo/chronic/lib/chronic/repeaters/repeater_quarter.rb:22: warning: instance variable @current_span not initialized
/Users/koic/src/github.com/mojombo/chronic/lib/chronic/repeaters/repeater_quarter.rb:22: warning: instance variable @current_span not initialized
/Users/koic/src/github.com/mojombo/chronic/lib/chronic/repeaters/repeater_quarter.rb:22: warning: instance variable @current_span not initialized
/Users/koic/src/github.com/mojombo/chronic/lib/chronic/repeaters/repeater_quarter_name.rb:11: warning: instance variable @current_span not initialized

(snip)