Work around rust-lang/cargo#4664

Our dev-dependency on trybuild enables serde's "derive" feature, which
we don't want affecting `cargo build`.

Author: dtolnay

.travis.yml

@@ -13,12 +13,18 @@ matrix:
     - rust: 1.15.0
       script:
         # preserve_order is not supported on 1.15.0
-        - cargo build
-        - cargo build --features arbitrary_precision
+        - cargo build --manifest-path tests/crate/Cargo.toml
+        - cargo build --manifest-path tests/crate/Cargo.toml --features arbitrary_precision
+
+    - rust: 1.18.0
+      script:
+        - cargo build --manifest-path tests/crate/Cargo.toml
+        - cargo build --manifest-path tests/crate/Cargo.toml --features preserve_order
+        - cargo build --manifest-path tests/crate/Cargo.toml --features arbitrary_precision
 
     - rust: stable
     - rust: beta
-    - rust: 1.18.0
+    - rust: 1.31.0
 
     - rust: nightly
       name: Clippy

tests/crate/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "serde_json_test"
+version = "0.0.0"
+edition = "2018"
+publish = false
+
+[lib]
+path = "test.rs"
+
+[dependencies]
+serde_json = { path = "../.." }
+
+[features]
+arbitrary_precision = ["serde_json/arbitrary_precision"]
+preserve_order = ["serde_json/preserve_order"]
+raw_value = ["serde_json/raw_value"]
+unbounded_depth = ["serde_json/unbounded_depth"]

tests/crate/test.rs

@@ -0,0 +1 @@
+pub use serde_json::*;