Increase TFP dev version 0.19.0-dev, now that TFP 0.18.0 has been released.

PiperOrigin-RevId: 473785944

Author: jburnim

tensorflow_probability/python/version.py

@@ -16,7 +16,7 @@
 
 # We follow Semantic Versioning (https://semver.org/)
 _MAJOR_VERSION = '0'
-_MINOR_VERSION = '18'
+_MINOR_VERSION = '19'
 _PATCH_VERSION = '0'
 
 # When building releases, we can update this value on the release branch to