Fix an AttributeError in the editable builder

CHANGELOG.md

 # Change Log
 
+## [Unreleased]
+
+### Fixed
+
+- Fixed an `AttributeError` in the editable builder.
+
+
 ## [0.12.14] - 2019-04-26
 
 ### Fixed

poetry/masonry/builders/editable.py

@@ -22,7 +22,7 @@ class EditableBuilder(Builder):
         has_setup = setup.exists()
 
         if has_setup:
-            self._io.write_line(
+            self._io.writeln(
                 "<warning>A setup.py file already exists. Using it.</warning>"
             )
         else: