Gemfile in neo4j-9.2.3 vs Gemfile in neo4j-9.2.4

@@ -2,13 +2,15 @@
 
 gemspec
 
 # gem 'neo4j-core', github: 'neo4jrb/neo4j-core', branch: 'master' if ENV['CI']
 
-branch = ENV['TRAVIS_PULL_REQUEST_BRANCH'] || ENV['TRAVIS_BRANCH']
-slug = ENV['TRAVIS_PULL_REQUEST_SLUG'] || ENV['TRAVIS_REPO_SLUG']
+branch = ENV['NEO4J_CORE_BRANCH'] || ENV['TRAVIS_PULL_REQUEST_BRANCH'] || ENV['TRAVIS_BRANCH']
+slug = !ENV['TRAVIS_PULL_REQUEST_SLUG'].to_s.empty? ? ENV['TRAVIS_PULL_REQUEST_SLUG'] : ENV['TRAVIS_REPO_SLUG']
 if branch
-  if `curl --head https://github.com/#{slug}-core/tree/#{branch} | head -1` =~ /200 OK/
+  command = "curl --head https://github.com/#{slug}-core/tree/#{branch} | head -1"
+  result = `#{command}`
+  if result =~ /200 OK/
     gem 'neo4j-core', github: "#{slug}-core", branch: branch
   else
     gem 'neo4j-core', github: 'neo4jrb/neo4j-core', branch: 'master'
   end
 elsif ENV['USE_LOCAL_CORE']