Make version a valid semver.

ntoll · ntoll · commit 92540aa8813b · 2019-09-06T17:58:03.000+01:00
diff --git a/adafruit_rsa/__init__.py b/adafruit_rsa/__init__.py
@@ -29,4 +29,4 @@
 
 __author__ = "Sybren Stuvel, Barry Mead and Yesudeep Mangalapilly"
 __date__ = "2018-09-16"
-__version__ = '4.0'
+__version__ = '4.0.0'
