Fix envvars to be strings

Change-Id: Ia777482a44cbcc3199fb71cd0712cd498e652730
diff --git a/turbo_hipster/lib/models.py b/turbo_hipster/lib/models.py
index 39d8b89..3987733 100644
--- a/turbo_hipster/lib/models.py
+++ b/turbo_hipster/lib/models.py
@@ -320,7 +320,7 @@
         return_code = 1
         while return_code != 0:
             tries += 1
-            env.update({'GIT_CURL_VERBOSE': 1, 'GIT_TRACE': 1})
+            env.update({'GIT_CURL_VERBOSE': '1', 'GIT_TRACE': '1'})
             return_code = utils.execute_to_log(cmd, self.git_prep_log,
                                                env=env, cwd=local_path)
             if tries == 2: