snf-pithos-backend: Change SQLAlchemy version

Change SQLAlchemy dependency version to match
with the one found in Debian wheezy.

snf-pithos-backend/setup.py

@@ -58,7 +58,7 @@ CLASSIFIERS = []
 # Package requirements
 INSTALL_REQUIRES = [
     'snf-common',
-    'SQLAlchemy==0.6.3',
+    'SQLAlchemy>=0.6.3, <=0.7.8',
     'alembic>=0.3.4, <0.4',
     'objpool>=0.2'
 ]