Add support for Ruby 2.7 and it's kwargs

[nbulaj]

Ruby 2.7.0 was released December 25, 2019, and has some noisy deprecation messages about keyword arguments.

acts-as-taggable-on calls multiple methods from ActiveRecord that have kwargs as the last argument and produce warnings to stdout.

So, what's done here:

• Add Ruby 2.7 to Travis matrix
• Get rid of kwargs deprecation messages from the gem