Switch md5 hashing for sha256

On FIPS[0] enabled systems unsafe cryptographic modules are disabled.
This commit changes out the md5 function for sha256, both of hashlib.

[0] http://csrc.nist.gov/groups/STM/cmvp/documents/140-1/140val-all.htm

compressor/cache.py

@@ -18,7 +18,7 @@ _cachekey_func = None
 
 
 def get_hexdigest(plaintext, length=None):
-    digest = hashlib.md5(smart_bytes(plaintext)).hexdigest()
+    digest = hashlib.sha256(smart_bytes(plaintext)).hexdigest()
     if length:
         return digest[:length]
     return digest