Fix dependencies spec and add CI testing with -Zminimal-versions

.travis.yml

@@ -19,6 +19,8 @@ script:
   - cargo build --verbose --no-default-features
   - CBINDGEN_TEST_VERIFY=1 cargo test --verbose
   - cargo build --verbose --release
+  - cargo update -Zminimal-versions
+  - cargo test
 deploy:
   provider: releases
   api_key:

Cargo.toml

@@ -17,7 +17,7 @@ travis-ci = { repository = "eqrion/cbindgen" }
 [dependencies]
 clap = { version = "2", optional = true }
 log = "0.4"
-serde = { version = "1.0", default-features = false, features = ["derive"]}
+serde = { version = "1.0.103", default-features = false, features = ["derive"]}
 serde_json = "1.0"
 tempfile = "3.0"
 toml = "0.5"
@@ -25,7 +25,7 @@ proc-macro2 = "1"
 quote = "1"
 
 [dependencies.syn]
-version = "1"
+version = "1.0.3"
 default-features = false
 features = ["clone-impls", "extra-traits", "full", "parsing", "printing"]