Fix tests

tests/installation/test_installer.py

 from __future__ import unicode_literals
 
+import sys
+
 import pytest
 import toml
 
@@ -81,6 +83,18 @@ class Locker(BaseLocker):
         self._written_data = data
 
 
+@pytest.fixture(autouse=True)
+def setup():
+    # Mock python version and platform to get reliable tests
+    original_platform = sys.platform
+
+    sys.platform = 'darwin'
+
+    yield
+
+    sys.platform = original_platform
+
+
 @pytest.fixture()
 def package():
     return get_package('root', '1.0')