Update serde_derive minimum rustc version to 1.31

.travis.yml

@@ -39,11 +39,6 @@ matrix:
         - cd "${TRAVIS_BUILD_DIR}/serde_test"
         - cargo build
 
-    - rust: 1.15.0
-      script:
-        - cd "${TRAVIS_BUILD_DIR}/serde_derive"
-        - cargo build
-
     - rust: 1.19.0
     - rust: 1.20.0
     - rust: 1.21.0
@@ -56,6 +51,11 @@ matrix:
     - rust: 1.28.0
       before_script: sed -i /test_suite/d Cargo.toml
 
+    - rust: 1.31.0
+      script:
+        - cd "${TRAVIS_BUILD_DIR}/serde_derive"
+        - cargo build
+
     - rust: 1.34.0
 
     - rust: 1.36.0