travis/rust: update rust minimum to 1.21

Ubuntu LTS and CentOS7/EPEL has upgraded to 1.21.

Update highest known working version to 1.23.

.travis.yml

@@ -81,24 +81,24 @@ matrix:
         - DO_CHECK_SETUP_SCRIPTS="yes"
         - DO_DISTCHECK="yes"
     # Linux, gcc, Rust.
-    # - Rust 1.21.0, the latest known working version.
+    # - Rust 1.23.0, the latest known working version.
     - os: linux
       compiler: gcc
       env:
-        - NAME="linux,gcc,rust-1.21.0"
+        - NAME="linux,gcc,rust-1.23.0"
         - *default-cflags
         - ENABLE_RUST="yes"
-        - RUST_VERSION="1.21.0"
+        - RUST_VERSION="1.23.0"
         - ARGS="--enable-rust --enable-rust-strict"
         - DO_DISTCHECK="yes"
-    # Linux, gcc, Rust (1.15.0 - oldest supported).
+    # Linux, gcc, Rust (1.21.0 - oldest supported).
     - os: linux
       compiler: gcc
       env:
-        - NAME="linux,gcc,rust-1.15.0"
+        - NAME="linux,gcc,rust-1.21.0"
         - *default-cflags
         - ENABLE_RUST="yes"
-        - RUST_VERSION="1.15.0"
+        - RUST_VERSION="1.21.0"
         - ARGS="--enable-rust --enable-rust-strict"
         - DO_DISTCHECK="yes"
     # Linux, gcc, -DNDEBUG.