Resolution of NameError (undefined local variable or method `doc')
Resolution of NoMethodError (undefined method `instance')
tests: switch to minitest

Ruby 1.9+ uses Minitest as the backend for Test::Unit. As of Minitest 5,
the shim no longer supports Test::Unit::TestCase.

Adjust the test suite to support Minitest 5's syntax.

Minitest versions 4 and below do not support the newer Minitest::Test
class that arrived in version 5. For that case, use the
MiniTest::Unit::TestCase class as a fallback.
Use defined? over const_defined to check for Minitest::Test
Merge pull request #937 from ktdreyer
add option to run-tests.sh script to run tests on all Rubies
lock minitest version to ~> 5.3.0
fix regexp match group false positives in Opal
another minor Opal fix in substitutions
switch to semver standard
set version to 1.5.0-preview.5
Release 1.5.0-preview.5
set version to 1.5.0.preview.5

apparently you can't use hyphens in a RubyGem version
Release 1.5.0.preview.5
prepare for next development iteration
Opal 0.6.2 compatibility fixes
set version to 1.5.0.preview.6
Release 1.5.0.preview.6
prepare for next development iteration
resolves #836 and #846

- use new Opal preprocessor guards (i.e., unless RUBY_ENGINE == 'opal')
- use unicode escape sequence for passthrough placeholders and null character
- fix several checks where a value is empty string instead of null in JavaScript
- consolidate opal_ext require to a single line
- move autoload declarations outside of Asciidoctor module block
- use qualified name for top-level classes in AbstractNode