Fix sub dependencies representation in lock file

Author: sdispater

CHANGELOG.md

@@ -7,7 +7,8 @@
 - Fixed built wheels not getting information from the virtualenv.
 - Fixed building wheel with conditional extensions.
 - Fixed missing files in built wheel with extensions.
-- Fixed call to venv binaries on windows
+- Fixed call to venv binaries on windows.
+- Fixed subdependencies representation in lock file.
 
 
 [0.6.2] - 2018-03-19

poetry/packages/locker.py

@@ -184,7 +184,7 @@ def _dump_package(self, package: 'poetry.packages.Package') -> dict:
             if dependency.is_optional():
                 continue
 
-            dependencies[dependency.pretty_name] = dependency.pretty_constraint
+            dependencies[dependency.pretty_name] = str(dependency.pretty_constraint)
 
         data = {
             'name': package.pretty_name,