Update `is-macro` to 0.1.9
Rust 1.53 introduced an intentional breaking change in attribute
handling in macros, and it led to is-macro
incorrectly generating code
for errors::HttpsWebhook::Tls
(investigated in rust-lang/rust#85692).
A fix for that landed in is-macro
0.1.9 via kdy1/is-macro#8, so tbot
needs to require is-macro
to be of version ^0.1.9
.