THRIFT-5457 Pin bitflags to 1.2 to maintain MSRV (#2454)

Client: rs

See https://issuehunt.io/r/clap-rs/clap/issues/2691 for an explanation of the underlying issue.
diff --git a/lib/rs/test/Cargo.toml b/lib/rs/test/Cargo.toml
index 9341bd5..c9ca298 100644
--- a/lib/rs/test/Cargo.toml
+++ b/lib/rs/test/Cargo.toml
@@ -8,6 +8,7 @@
 
 [dependencies]
 clap = "2.33"
+bitflags = "=1.2"
 
 [dependencies.thrift]
 path = "../"