Bumping to Release v.0.4.0

Features include:
New support for a Host SSH Certificate Lambda.  Please consider how you will control who can obtain host certs for which hostnames before using.
Updated publishing code to build with the latest Amazon Linux 2.
Validated for Python 3.7 Lambda runtime.
Updated dependencies.
Various typo fixes.

bless/__about__.py

@@ -9,7 +9,7 @@
     "sign SSH public keys.")
 __uri__ = "https://github.com/Netflix/bless"
 
-__version__ = "0.3.0"
+__version__ = "0.4.0"
 
 __author__ = "The BLESS developers"
 __email__ = "[email protected]"