Long overdue bump of TFP dev version number to 0.18

PiperOrigin-RevId: 472825747

Author: jburnim

tensorflow_probability/python/version.py

@@ -16,7 +16,7 @@
 
 # We follow Semantic Versioning (https://semver.org/)
 _MAJOR_VERSION = '0'
-_MINOR_VERSION = '17'
+_MINOR_VERSION = '18'
 _PATCH_VERSION = '0'
 
 # When building releases, we can update this value on the release branch to