Update extra_requires with recent changes fix #389

As per PR comment:
- remove deprecated backend
- correct extra require for google cloud
- update minimum version for dropbox

setup.py

@@ -19,9 +19,8 @@ def get_requirements_tests():
         'azure': ['azure'],
         'boto': ['boto>=2.32.0'],
         'boto3': ['boto3>=1.2.3'],
-        'database': ['pyodbc'],
-        'dropbox': ['dropbox>=3.24'],
-        'google': ['boto>=2.32.0'],
+        'dropbox': ['dropbox>=7.2.1'],
+        'google': ['google-cloud-storage>=0.22.0'],
         'libcloud': ['apache-libcloud'],
         'sftp': ['paramiko'],
     },