Convert all S3 endpoint representations to unicode

To fix `tox -e v0.9-oldest` builds, as *old* boto versions used a str
representation, and this would overwrite the unicode in the in-WAL-E
endpoint dictionary, and new versions use unicode (that WAL-E also
adapted).

wal_e/blobstore/s3/calling_format.py

@@ -26,7 +26,7 @@
     # Override the hard-coded region map with boto's mappings if
     # available.
     from boto.s3 import regions
-    _S3_REGIONS.update(dict((r.name, r.endpoint) for r in regions()))
+    _S3_REGIONS.update(dict((r.name, unicode(r.endpoint)) for r in regions()))
 except ImportError:
     pass