Fix to remove wanings for unary/binary operators

[vassilios]

Fix to remove these warnings in jruby 9.1.5.0:

/opt/jruby-9.1.5.0/lib/ruby/gems/shared/gems/savgol-0.4.0/lib/savgol.rb:27: warning: `-' after local variable or literal is interpreted as binary operator
/opt/jruby-9.1.5.0/lib/ruby/gems/shared/gems/savgol-0.4.0/lib/savgol.rb:27: warning: even though it seems like unary operator
/opt/jruby-9.1.5.0/lib/ruby/gems/shared/gems/savgol-0.4.0/lib/savgol.rb:107: warning: `-' after local variable or literal is interpreted as binary operator
/opt/jruby-9.1.5.0/lib/ruby/gems/shared/gems/savgol-0.4.0/lib/savgol.rb:107: warning: even though it seems like unary operator