Fix the build; modernize gemspec, more compatible Gemfile

[olleolleolle]

TL;DR: This PR fixes the build ✨

Link to passing build

Details

gemspec:

• updated the gemspec ERB template
  • dropped unsupported & deprecated RubyGems settings
• regenerated the gemspec
  • avoided changing the date
  • updated the list of test files, to match what is in the repo now

Bundler:

• removed BUNDLED_WITH in lockfile,
• hand-picked v1.17.3 for 2.2+ compatibility
• Updated gems rake 13+ and test-unit to avoid warnings in ruby-head

CI:

• update to Rubinius 4.x:
  • name dist: trusty to make Rubinius even build; example of legitimate test failure in rbx-3, mixlib-shellout uses Marshal.dump/Marshal.load
  • update the Gemfile to reflect the fact that Rubinius now ships with a full set of standard libraries

Tests:

• 🍏 one test was failing in ruby-head: made a space optional at the end of an assert_match check (see same fix in Update tc_curl_easy.rb #406 and marked as a problem in unwanted space in tests/tc_curl_easy.rb? (line 24) #405)

---

The warning in CI which made me look at the gemspec (which led to all this adventure!)

NOTE: Gem::Specification#has_rdoc= is deprecated with no replacement. It will be removed on or after 2018-12-01.

Gem::Specification#has_rdoc= called from /home/travis/build/taf2/curb/curb.gemspec:22.

The RubyGems gemspec property rubyforge_project has been removed without a replacement. This PR removes that property. RubyForge was closed down in 2013, rubygems/rubygems#2436 deprecated the rubyforge_project property.

[olleolleolle]

One of the effects of this PR was "fixes #405".

That makes this PR a partial duplicate of #406

(Worked that into the description field.)

[olleolleolle]

If you would like fewer commits, @taf2, I can rebase them in a shorter sequence.

[taf2]

Thanks merged!