Add development dependency on rexml, which seems to be needed for ruby-head and is used for webmock/crack.

Author: gkellogg

.travis.yml

@@ -7,10 +7,12 @@ rvm:
   - 2.5
   - 2.6
   - 2.7
+  - ruby-head
   - jruby
 cache: bundler
 sudo: false
 matrix:
   allow_failures:
     - rvm: jruby
+    - rvm: ruby-head
 dist: trusty

rdf-spec.gemspec

@@ -25,6 +25,7 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency     'rdf-isomorphic',  '~> 3.1'
   gem.add_runtime_dependency     'rspec',           '~> 3.9'
   gem.add_runtime_dependency     'rspec-its',       '~> 1.3'
+  gem.add_development_dependency 'rexml',           '~> 3.2'  # For Webmock
   gem.add_runtime_dependency     'webmock',         '~> 3.7'
   gem.add_development_dependency 'yard' ,           '~> 0.9.20'
   gem.post_install_message        = nil