infra: Refactor travis.

* Use build stages to be clearer.
 * Run Python / C++ tests / Format separately.
 * Reduce duplication

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

.travis.yml

@@ -1,43 +1,33 @@
-matrix:
+os: linux
+dist: trusty
+language: python
+python: 3.6
+addons:
+  apt:
+    sources:
+      - ubuntu-toolchain-r-test
+      - llvm-toolchain-trusty-5.0
+    packages:
+      - clang-format-5.0
+      - g++-6
+
+install:
+  - export CC=gcc-6
+  - export CXX=g++-6
+  - make env
+
+
+jobs:
   include:
-    # Job 1) Runs the tests
-    - os: linux
-      dist: trusty
-      language: python
-      python: 3.6
-      addons:
-        apt:
-          sources:
-            - ubuntu-toolchain-r-test
-          packages:
-            - g++-6
-      install:
-        - export CC=gcc-6
-        - export CXX=g++-6
-        - make env
+    # State 1 - Tests
+    - stage: "Tests"
+      name: "C++ Tests"
+      script: make test-cpp
+
+    - name: "Python Tests"
+      script: make test-py
+
+    - name: "Format"
       script:
-        - export CC=gcc-6
-        - export CXX=g++-6
-        - make test
-    # Job 2) Checks the formatting
-    - os: linux
-      dist: trusty
-      language: python
-      python: 3.6
-      addons:
-        apt:
-          sources:
-            - llvm-toolchain-trusty-5.0
-            - ubuntu-toolchain-r-test
-          packages:
-            - clang-format-5.0
-            - g++-6
-      install:
-        - export CC=gcc-6
-        - export CXX=g++-6
-        - make env
-      script:
-        - export CC=gcc-6
-        - export CXX=g++-6
         - make format
         - test $(git status --porcelain | wc -l) -eq 0 || { git diff; false; }