Prepare 0.1.6 Release

This PR updates the `RELEASES.md` to capture the changes introduced in #319 and #329. Thanks to @djc and @infinity0 for their help!

Author: davidbarsky

RELEASES.md

@@ -1,3 +1,8 @@
+# Version 0.1.6
+
+- Update `syn`, `quote`, and `proc_macro2` dependencies to 1.0.
+- Bump MSRV to 1.31.0.
+
 # Version 0.1.5
 
 - Resolve a regression with error conversions (#290)