travis: update rust to 1.29.1, add auto & disabled tests

.travis.yml

@@ -84,15 +84,37 @@ matrix:
         - ARGS="--enable-rust --enable-rust-strict"
         - DO_CHECK_SETUP_SCRIPTS="yes"
         - DO_DISTCHECK="yes"
+    # Linux, gcc, Rust (auto detect).
+    # - Rust 1.29.1, the latest known working version.
+    - os: linux
+      compiler: gcc
+      env:
+        - NAME="linux,gcc,rust-1.29.1-disabled"
+        - *default-cflags
+        - ENABLE_RUST="yes"
+        - RUST_VERSION="1.29.1"
+        - ARGS="--disable-rust"
+        - DO_DISTCHECK="yes"
+    # Linux, gcc, Rust (auto detect).
+    # - Rust 1.29.1, the latest known working version.
+    - os: linux
+      compiler: gcc
+      env:
+        - NAME="linux,gcc,rust-1.29.1-auto"
+        - *default-cflags
+        - ENABLE_RUST="yes"
+        - RUST_VERSION="1.29.1"
+        - ARGS=""
+        - DO_DISTCHECK="yes"
     # Linux, gcc, Rust.
-    # - Rust 1.23.0, the latest known working version.
+    # - Rust 1.29.1, the latest known working version.
     - os: linux
       compiler: gcc
       env:
-        - NAME="linux,gcc,rust-1.23.0"
+        - NAME="linux,gcc,rust-1.29.1"
         - *default-cflags
         - ENABLE_RUST="yes"
-        - RUST_VERSION="1.23.0"
+        - RUST_VERSION="1.29.1"
         - ARGS="--enable-rust --enable-rust-strict"
         - DO_DISTCHECK="yes"
     # Linux, gcc, Rust (1.21.0 - oldest supported).