travis: update rust version to 1.24.1 and 1.31.0.

1.24.1 is now the oldest version we test support for. All major
distributions appear to be at this version or new.

With the release of 1.31.0 just out, test that as the most
recent version.

.travis.yml

@@ -85,46 +85,46 @@ matrix:
         - DO_CHECK_SETUP_SCRIPTS="yes"
         - DO_DISTCHECK="yes"
     # Linux, gcc, Rust (auto detect).
-    # - Rust 1.29.1, the latest known working version.
+    # - Rust 1.31.0, the latest known working version.
     - os: linux
       compiler: gcc
       env:
-        - NAME="linux,gcc,rust-1.29.1-disabled"
+        - NAME="linux,gcc,rust-1.31.0-disabled"
         - *default-cflags
         - ENABLE_RUST="yes"
-        - RUST_VERSION="1.29.1"
+        - RUST_VERSION="1.31.0"
         - ARGS="--disable-rust"
         - DO_DISTCHECK="yes"
     # Linux, gcc, Rust (auto detect).
-    # - Rust 1.29.1, the latest known working version.
+    # - Rust 1.31.0, the latest known working version.
     - os: linux
       compiler: gcc
       env:
-        - NAME="linux,gcc,rust-1.29.1-auto"
+        - NAME="linux,gcc,rust-1.31.0-auto"
         - *default-cflags
         - ENABLE_RUST="yes"
-        - RUST_VERSION="1.29.1"
+        - RUST_VERSION="1.31.0"
         - ARGS=""
         - DO_DISTCHECK="yes"
     # Linux, gcc, Rust.
-    # - Rust 1.29.1, the latest known working version.
+    # - Rust 1.31.0, the latest known working version.
     - os: linux
       compiler: gcc
       env:
-        - NAME="linux,gcc,rust-1.29.1"
+        - NAME="linux,gcc,rust-1.31.0"
         - *default-cflags
         - ENABLE_RUST="yes"
-        - RUST_VERSION="1.29.1"
+        - RUST_VERSION="1.31.0"
         - ARGS="--enable-rust --enable-rust-strict"
         - DO_DISTCHECK="yes"
-    # Linux, gcc, Rust (1.21.0 - oldest supported).
+    # Linux, gcc, Rust (1.24.1 - oldest supported).
     - os: linux
       compiler: gcc
       env:
-        - NAME="linux,gcc,rust-1.21.0"
+        - NAME="linux,gcc,rust-1.24.1"
         - *default-cflags
         - ENABLE_RUST="yes"
-        - RUST_VERSION="1.21.0"
+        - RUST_VERSION="1.24.1"
         - ARGS="--enable-rust --enable-rust-strict"
         - DO_DISTCHECK="yes"
     # Linux, gcc, -DNDEBUG.