Change the project status badge to "passively-maintained"

This crate provides low-level features and they seem to be working well.
I'll maintain this crate (to add features, fix bugs, refactor, and
support more data), but currently I have no plan to add new features
or change the API drastically.

Cargo.toml

@@ -30,7 +30,7 @@ string-interner = { version = "0.7", optional = true, default-features = false }
 env_logger = "0.7"
 
 [badges]
-maintenance = { status = "actively-developed" }
+maintenance = { status = "passively-maintained" }
 travis-ci = { repository = "lo48576/fbxcel" }
 
 [[example]]