Reorder and clean up travis build config to make the most interesting jobs run...

Reorder and clean up travis build config to make the most interesting jobs run first: Py2.7/3.7/2.6.

.travis.yml

 os: linux
 dist: trusty
+language: python
 # 'sudo' is enabled automatically by the 'apt' addon below.
 #sudo: false
 
@@ -18,14 +19,6 @@ cache:
   directories:
     - $HOME/.ccache
 
-language: python
-python:
-  - 2.7
-  - 3.6
-  - 2.6
-  - 3.4
-  - 3.5
-
 env:
   global:
     - USE_CCACHE=1
@@ -34,16 +27,14 @@ env:
     - CCACHE_MAXSIZE=250M
     - PATH="/usr/lib/ccache:$HOME/miniconda/bin:$PATH"
     - BACKEND=c,cpp
-  #matrix:
-  #  - BACKEND=c
-  #  - BACKEND=cpp
 
 matrix:
   include:
+    - python: 2.7
     - python: 3.7
       dist: xenial    # Required for Python 3.7
       sudo: required  # travis-ci/travis-ci#9069
-      env: TEST_CODE_STYLE=1
+    - python: 2.6
     - python: 3.7
       dist: xenial    # Required for Python 3.7
       sudo: required  # travis-ci/travis-ci#9069
@@ -51,49 +42,27 @@ matrix:
     - python: 3.7
       dist: xenial    # Required for Python 3.7
       sudo: required  # travis-ci/travis-ci#9069
-#      env: BACKEND=c
-#    - python: 3.7
-#      dist: xenial    # Required for Python 3.7
-#      sudo: required  # travis-ci/travis-ci#9069
-#      env: BACKEND=cpp
+      env: TEST_CODE_STYLE=1
+    - python: 3.4
+    - python: 3.5
+    - python: 3.6
     - python: 3.8-dev
       dist: xenial    # Required for Python 3.7
       sudo: required  # travis-ci/travis-ci#9069
-#      env: BACKEND=c
-#    - python: 3.8-dev
-#      dist: xenial    # Required for Python 3.7
-#      sudo: required  # travis-ci/travis-ci#9069
-#      env: BACKEND=cpp
     - os: osx
       osx_image: xcode6.4
-#      env: BACKEND=c PY=2
       env: PY=2
       python: 2
       language: c
       compiler: clang
       cache: false
-#    - os: osx
-#      osx_image: xcode6.4
-#      env: BACKEND=cpp PY=2
-#      python: 2
-#      language: cpp
-#      compiler: clang
-#      cache: false
     - os: osx
       osx_image: xcode6.4
-#      env: BACKEND=c PY=3
       env: PY=3
       python: 3
       language: c
       compiler: clang
       cache: false
-#    - os: osx
-#      osx_image: xcode6.4
-#      env: BACKEND=cpp PY=3
-#      python: 3
-#      language: cpp
-#      compiler: clang
-#      cache: false
     - python: pypy
       env: BACKEND=c
     - python: pypy3
@@ -108,11 +77,6 @@ matrix:
     - python: 3.8-dev
     - env: STACKLESS=true BACKEND=c PY=2
     - env: STACKLESS=true BACKEND=c PY=3
-#  exclude:
-#    - python: pypy
-#      env: BACKEND=cpp
-#    - python: pypy3
-#      env: BACKEND=cpp
 
 branches:
   only: