Add support for the GCLOUD_PROJECT environment variable (#73)

diff --git a/google/auth/_default.py b/google/auth/_default.py
index 356780b..b6014e7 100644
--- a/google/auth/_default.py
+++ b/google/auth/_default.py
@@ -254,7 +254,9 @@
             If no credentials were found, or if the credentials found were
             invalid.
     """
-    explicit_project_id = os.environ.get(environment_vars.PROJECT)
+    explicit_project_id = os.environ.get(
+        environment_vars.PROJECT,
+        os.environ.get(environment_vars.LEGACY_PROJECT))
 
     checkers = (
         _get_explicit_environ_credentials,