travis: add rust 1.7.0 build

One build with Rust 1.7.0, our oldest that we'll support as its
whats bundled with Ubuntu 16.04. Create another build that will use
the latest stable.

.travis.yml

@@ -47,10 +47,8 @@ matrix:
     - os: linux
       compiler: gcc
       env:
-        - NAME="linux,gcc,cocci,rust"
+        - NAME="linux,gcc,cocci"
         - *default-cflags
-        - ENABLE_RUST="yes"
-        - ARGS="--enable-rust"
       addons:
         apt:
           sources:
@@ -58,6 +56,23 @@ matrix:
           packages:
             - *packages
             - coccinelle
+    # Linux, gcc, Rust (latest stable).
+    - os: linux
+      compiler: gcc
+      env:
+        - NAME="linux,gcc,rust"
+        - *default-cflags
+        - ENABLE_RUST="yes"
+        - ARGS="--enable-rust"
+    # Linux, gcc, Rust (1.7.0 - oldest supported).
+    - os: linux
+      compiler: gcc
+      env:
+        - NAME="linux,gcc,rust-1.7.0"
+        - *default-cflags
+        - ENABLE_RUST="yes"
+        - RUST_VERSION="1.7.0"
+        - ARGS="--enable-rust"
     # Linux, gcc, -DNDEBUG.
     - os: linux
       compiler: gcc
@@ -139,11 +154,15 @@ script:
     fi
 
 before_install:
+  - export PATH=$HOME/.cargo/bin:$PATH
   - |
     if [[ "$ENABLE_RUST" == "yes" ]]; then
         curl https://sh.rustup.rs -sSf | sh -s -- -y
+        if [[ "$RUST_VERSION" != "" ]]; then
+            rustup override set $RUST_VERSION
+        fi
+        rustc --version
     fi
-  - export PATH=$HOME/.cargo/bin:$PATH
   - |
     if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
         brew update