Tests on Ruby 2.7.0: avoid warnings; add 2.7.0 to CI

[jylitalo]

Fixing tests to work with ruby 2.7.0 (also tested with ruby 2.6.3 and 2.5.1)

With current master, we stumble into following error messages on ruby 2.7.0:

[~/github/slop]% rake test                                                                   [master]
/Users/jylitalo/github/slop/test/types_test.rb:42: warning: ambiguous first argument; put parentheses or a space even after `-' operator
Run options: --seed 59555

# Running:

/Users/jylitalo/github/slop/lib/slop/options.rb:32: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/Users/jylitalo/github/slop/lib/slop/parser.rb:13: warning: The called method `initialize' is defined here
/Users/jylitalo/github/slop/test/options_test.rb:124: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/Users/jylitalo/github/slop/lib/slop/options.rb:27: warning: The called method `initialize' is defined here
./Users/jylitalo/github/slop/test/options_test.rb:128: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/Users/jylitalo/github/slop/lib/slop/options.rb:27: warning: The called method `initialize' is defined here
./Users/jylitalo/github/slop/lib/slop/options.rb:85: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/Users/jylitalo/github/slop/lib/slop/options.rb:51: warning: The called method `on' is defined here
/Users/jylitalo/github/slop/lib/slop/options.rb:55: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/Users/jylitalo/github/slop/lib/slop/option.rb:30: warning: The called method `initialize' is defined here
.............................................................................../Users/jylitalo/github/slop/test/option_test.rb:5: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
/Users/jylitalo/github/slop/lib/slop/option.rb:30: warning: The called method `initialize' is defined here
....

Finished in 0.014476s, 5871.7877 runs/s, 8151.4230 assertions/s.

85 runs, 118 assertions, 0 failures, 0 errors, 0 skips
[~/github/slop]%

Closes #247 - thanks for that!

[olleolleolle]

Thanks for maintaining a zero-warning state!

Add 2.7.0 to CI, with a cherry-picked commit from the PR @ankane offered yesterday. Thanks for that, you!

[ankane]

Awesome, thank you both!