Support Ruby >= 2.0.0

[arielj]

This PR adds support for older Ruby 2 versions.

Things I changed:

• added explicit default values for keyword arguments (implicit values not supported in older rubies)
• limit rexml version, newer versions use implicit default values for keywords arguments (related to the previous change)
• use the <<- heredoc syntax instead of <<~, the ~ version didn't exist in older rubies (since what it does is remove the indentation, I removed the indentation in the code instead)
• use Object.extend if Object.prepend is not a valid method, it was added in Ruby 2.1.10 https://apidock.com/ruby/v2_1_10/Module/prepend
• replace a call to Array#to_h, since that method was added in Ruby 2.1.10 https://apidock.com/ruby/v2_1_10/Array/to_h
• configured the github actions to run the tests with more Ruby versions

I tried to add support for ruby 1.9.3 but it requires rewriting methods because the ruby2 keyword arguments syntax is not valid. Adding this for reference https://bugs.ruby-lang.org/issues/7664

[kindoflew]

hello @aisayo! welcome to your first OSS request for review!

whenever you have a moment, could you review this PR (you can log it under the Open Source project in Noko and it will count towards your monthly OSS goal!)

thanks!

[kindoflew]

ping @aisayo 😃

[bronzdoc]

@arielj can you rebase the changelog