Add "this is an issue on stable" to our bug report template

I really doubt this will help, but we should at least try it...

Author: sgrif

.github/ISSUE_TEMPLATE.md

@@ -41,7 +41,7 @@ Please post as much of your database schema as necessary. If you are using `infe
 ## Checklist
 
 - [ ] I have already looked over the [issue tracker](https://github.com/diesel-rs/diesel/issues) for similar issues.
-
+- [ ] This issue can be reproduced on Rust's stable channel.
 
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