Add Rust 1.31.1 as fixed build target to Travis-CI

.travis.yml

@@ -25,14 +25,23 @@ matrix:
         - cd plugin
         - luacov-coveralls -e $TRAVIS_BUILD_DIR/lua_install
 
+    - language: rust
+      rust: 1.31.1
+      cache: cargo
+
+      script:
+        - cargo test --verbose
+
     - language: rust
       rust: stable
+      cache: cargo
 
       script:
         - cargo test --verbose
 
     - language: rust
       rust: beta
+      cache: cargo
 
       script:
         - cargo test --verbose