Use heredoc now that we can count on Ruby 2.3+

lib/beaker/hypervisor/vagrant.rb

@@ -228,9 +228,8 @@ def provision(provider = nil)
       # make sure that any old boxes are dead dead dead
       begin
         vagrant_cmd('destroy --force') if File.file?(@vagrant_file)
-      rescue RuntimeError => e
-        # LATER: use <<~MESSAGE once we're on Ruby 2.3
-        @logger.debug(%(
+      rescue
+        @logger.debug <<~MESSAGE
           Beaker failed to destroy the existing VM's. If you think this is
           an error or you upgraded from an older version of beaker try
           verifying the VM exists and deleting the existing Vagrantfile if
@@ -240,8 +239,8 @@ def provision(provider = nil)
           vagrant status
           vagrant destroy # only need to run this is a VM is not created
           rm #{@vagrant_file} # only do this if all VM's are actually destroyed
-        ).each_line.map(&:strip).join("\n"))
-        raise e
+        MESSAGE
+        raise
       end
 
       make_vfile @hosts, @options