Fix cucumber puts deprecation warnings (capistrano#2075)

Cucumber has deprecated `puts` in favor of `log` for logging messages to
the current cucumber formatter. In our case we actually want it both
ways: we want to log messages using the cucumber formatter when cucumber
is running, but use `Kernel#puts` otherwise.

So, use `respond_to?` to see if cucumber's `log` is available, and if
not, fall back to `puts`.

features/support/vagrant_helpers.rb

@@ -30,6 +30,12 @@ def run_vagrant_command(command)
     return [stdout, stderr] if status.success?
     raise VagrantSSHCommandError, status
   end
+
+  def puts(message)
+    # Attach log messages to the current cucumber feature (`log`),
+    # or simply puts to the console (`super`) if we are outside of cucumber.
+    respond_to?(:log) ? log(message) : super(message)
+  end
 end
 
 World(VagrantHelpers)