Bump Cargo version number for 0.16.0 release

Author: mthjones

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "slack_api"
-version = "0.15.0"
+version = "0.16.0"
 authors = ["Benjamin Elder <ben.the.elder@gmail.com>", "Matt Jones <mthjones@gmail.com>"]
 repository = "https://github.com/slack-rs/slack-rs-api.git"
 documentation = "http://slack-rs.github.io/slack-rs-api/slack_api/"