Update changelog for 0.18.0

CHANGELOG.md

-## 0.18.0rc1 / 2016-04-07
+## 0.18.0 / 2016-04-18
 * [BUGFIX] Fix operator precedence in PromQL
+* [BUGFIX] Never drop still open head chunk
+* [BUGFIX] Fix missing 'keep_common' when printing AST node
 * [CHANGE/BUGFIX] Target identity considers path and parameters additionally to host and port
 * [CHANGE] Rename metric `prometheus_local_storage_invalid_preload_requests_total` to `prometheus_local_storage_non_existent_series_matches_total`
 * [CHANGE] Support for old alerting rule syntax dropped