feat: Turn serde into a default feature.

This is actually more of a workaround that I don't mind.  There isn't a
low maintenance way to enable a feature for tests.  I misunderstood
cargo's documentation and thought I could add a `[[test]]` section but
that instead disabled all the tests.

Fixes #128

.travis.yml

@@ -17,11 +17,11 @@ script:
   - cargo -V
   - cargo check --verbose
   - cargo check --verbose --features "cli"
-  - cargo check --verbose --features "cli serde serde_yaml serde_json"
-  - cargo test  --verbose --features "cli serde serde_yaml serde_json"
+  - cargo check --verbose --features "cli serde_yaml serde_json"
+  - cargo test  --verbose --features "cli serde_yaml serde_json"
   - ! which cargo-clippy || cargo when --channel nightly clippy -- --version
-  - ! which cargo-clippy || cargo when --channel nightly clippy --features "cli serde serde_yaml serde_json"
-  - cargo when --channel nightly bench --features "cli serde serde_yaml serde_json"
+  - ! which cargo-clippy || cargo when --channel nightly clippy --features "cli serde_yaml serde_json"
+  - cargo when --channel nightly bench --features "cli serde_yaml serde_json"
   - cargo when --channel stable doc --no-deps --all-features
   - cargo when --channel stable fmt -- --version
   - cargo when --channel stable fmt -- --write-mode=diff

Cargo.toml

@@ -24,12 +24,8 @@ doctest = false
 bench = false
 doc = false
 
-[[test]]
-name = "liquid"
-required-features = ["serde"]
-
 [features]
-default = ["extra-filters"]
+default = ["extra-filters", "serde"]
 cli = ["clap", "error-chain", "serde_yaml"]
 extra-filters = []
 dev = []
@@ -50,6 +46,7 @@ serde_json = { version = "1.0", optional = true }
 
 [build-dependencies]
 skeptic = "0.9"
+serde = { version = "1.0", features = ["derive"] }
 
 [dev-dependencies]
 difference = "1.0"