Close #170.

Problem

Since June 15th, 2021, the building on travis-ci.org is ceased.

Solution

This PR migrates CI from Travis CI to GitHub Actions.
Here are the details:

• Add Ruby 3.0 and 3.1 to the CI matrix

  • rbx-2 is not included because ruby/setup-ruby does not support it.

• Remove Coveralls

  • Because, according to 0.9.7 #161 (comment), it is not already in use.

• Address changes in RR v3.0.0

  • To use RR v3, we need to take the block as a proc, not as an argument.
    refs: rr/rr@d6da209#diff-a20b4cc1aac26c32a40206ad0776d4ff528201e97b737b7b3f0be1eb0e12e93dL44-L49

• Require OpenStruct v0.3.3+

  • Some test cases failed when running CI in Ruby 3.0.
  Details

     1) Failure:
   Twurl::CLI::OptionParsingTest#test_setting_host_updates_to_requested_value [/home/runner/work/twurl/twurl/test/cli_test.rb:214]:
   Expected: "localhost:3000"
     Actual: "api.twitter.com"
  
     2) Failure:
   Twurl::CLI::OptionParsingTest#test_setting_proxy_updates_to_requested_value [/home/runner/work/twurl/twurl/test/cli_test.rb:237]:
   Expected: "localhost:80"
     Actual: nil
  
     3) Failure:
   Twurl::CLI::OptionParsingTest#test_passing_no_ssl_option_disables_ssl [/home/runner/work/twurl/twurl/test/cli_test.rb:196]:
   Expected false to be truthy.
  
     4) Failure:
   Twurl::CLI::OptionParsingTest#test_specifying_a_request_method_extracts_and_normalizes_request_method [/home/runner/work/twurl/twurl/test/cli_test.rb:54]:
   Expected: "put"
     Actual: "get"
  
     5) Failure:
   Twurl::CLI::OptionParsingTest#test_protocol_is_stripped_from_host [/home/runner/work/twurl/twurl/test/cli_test.rb:221]:
   Expected: "localhost:3000"
     Actual: "api.twitter.com"
  
     6) Failure:
   Twurl::CLI::OptionParsingTest#test_passing_data_and_an_explicit_request_method_uses_the_specified_method [/home/runner/work/twurl/twurl/test/cli_test.rb:148]:
   Expected: "delete"
     Actual: "post"
  
     7) Failure:
   Twurl::Options::Test#test_ssl_is_enabled_if_the_protocol_is_https [/home/runner/work/twurl/twurl/test/cli_options_test.rb:18]:
   Expected false to be truthy.
  
     8) Failure:
   Twurl::Options::Test#test_base_url_is_built_from_protocol_and_host [/home/runner/work/twurl/twurl/test/cli_options_test.rb:13]:
   Expected: "http://api.twitter.com"
     Actual: "https://api.twitter.com"
  

  • According to INVESTIGATE: compatibility with Ruby 3 #159 (comment), the cause appears to be a bug in OpenStruct.
  • And the bug was fixed in Included module method are overridden  ruby/ostruct#23, and released as v0.3.3.
    @smaeda-ks Thanks for letting me know!

• Drop support for Ruby 2.4

  • OpenStruct v0.3.3+ requires Ruby 2.5+.
  • Ruby 2.4 has already reached EOL on 2020-03-31.
    (Ruby 2.5 and 2.6 has reached EOL also, though)

• Replace the CI status badge on README.md

The CI status can be checked here.