Make rexml gem an optional

See #146

Author: inukshuk

Gemfile

@@ -6,6 +6,8 @@ gem 'json', '~>2.0', platforms: %i[mri_18 jruby]
 gem 'rdf', '~>3.0'
 gem 'rdf-vocab', '~>3.0'
 
+gem 'rexml', '~>3.0'
+
 group :debug do
   gem 'byebug', require: false, platforms: :mri
   gem 'ruby-debug', require: false, platforms: :jruby

Gemfile.lock

@@ -3,7 +3,6 @@ PATH
   specs:
     bibtex-ruby (5.1.6)
       latex-decode (~> 0.0)
-      rexml (~> 3.0)
 
 GEM
   remote: https://rubygems.org/
@@ -124,6 +123,7 @@ DEPENDENCIES
   rdf (~> 3.0)
   rdf-vocab (~> 3.0)
   redcarpet
+  rexml (~> 3.0)
   ruby-debug
   ruby-prof
   simplecov

bibtex-ruby.gemspec

@@ -26,7 +26,6 @@ Gem::Specification.new do |s|
   END_DESCRIPTION
 
   s.add_runtime_dependency('latex-decode', ['~>0.0'])
-  s.add_runtime_dependency('rexml', ['~>3.0'])
 
   s.files        = File.open('Manifest').readlines.map(&:chomp)
   s.test_files   = Dir.glob('test/**/test*.rb')