fix required minimum indexmap version

nlordell · nlordell · commit 19ba7be97082 · 2019-12-04T11:31:54.000+01:00
diff --git a/Cargo.toml b/Cargo.toml
@@ -17,7 +17,7 @@ appveyor = { repository = "serde-rs/json" }
 
 [dependencies]
 serde = "1.0.60"
-indexmap = { version = "1.0", optional = true }
+indexmap = { version = "1.2", optional = true }
 itoa = "0.4.3"
 ryu = "1.0"
 
