Initial support for DBSubnetGroup
_sign_string private_key_file can now be file-like

Distribution._sign_string is now a little bit more flexible, allowing
the private_key_file to be a file-like object (or really anything with a
.read() method).
Added support for block device mappings in autoscaling groups (re #753, #1343)
updat incorrect copyright
Add missing function parameter (re: #1357)
add integration test
cloudformation Stack DisableRollback is always True

Fix problem where DisableRollback is always true, because
bool("False") == True
travis is secure by default now
Adding updated values for recurring autoscale operations
Adding updated documentation for the new variables
added docs for param_group in rds modify
Merge branch 'release-2.9.0' into develop

* release-2.9.0:
  Bump version to 2.9.0
Add dev prefix back to version in dev branch
Bumped the version in README.
Added back get_upload_id().
Merge pull request #1450 from reinhillmann/manifestchanges

Added back get_upload_id().
Fixing bogus docs regarding return value of import_key_pair.
Add eu-west-1 endpoint for Redshift.
Change calling_format override in storage_uri to be gs-specific.
Fixed missing raise introduced by 57a41897493e3b11fbbe47360a9d2f1033aae87d (fixes resumable download test failures)