Merge #13729: travis: Avoid unnecessarily setting env variables on the lint build

751c9587d4 Use the Travis python language feature on the lint build (Ben Woosley)
515348f704 Don't unnecessarily install shellcheck on the lint build (Ben Woosley)
1be5c33333 Avoid unnecessarily setting env variables on the lint build (Ben Woosley)

Pull request description:

  The relevent env variables are set for the matrix builds, and are irrelevant
  to the lint build. By default the first matrix entry is applied.

  "Each job included in jobs.include inherits the first value of the array that defines a matrix dimension."
  https://docs.travis-ci.com/user/build-stages/#Build-Stages-and-Build-Matrix-Expansion

  Note the global env variables are still applied, just the matrix are excluded:
  https://travis-ci.org/bitcoin/bitcoin/jobs/406455565#L558

  Before:
  <img width="755" alt="screen shot 2018-07-20 at 19 16 45" src="https://user-images.githubusercontent.com/5470/43029041-81e5fd14-8c51-11e8-9e2a-6c4bcbb36d2c.png">

  After:
  <img width="702" alt="screen shot 2018-07-20 at 19 23 05" src="https://user-images.githubusercontent.com/5470/43029187-5ec76b28-8c52-11e8-8b03-5bee859c0d96.png">

Tree-SHA512: 93bdf4da81e150d3eb90be1d94c4e9e90a5b38286049ffdad25036cbe6357409466e52ff4cb7f69f51af706119a6a9bfa595e8c240056bcfc0e1d0d898fd3178

.travis.yml

@@ -81,15 +81,13 @@ after_script:
 jobs:
   include:
     - stage: lint
+      env:
       sudo: false
       cache: false
-      addons:
-        apt:
-          packages:
-            - python3-pip
-            - shellcheck
+      language: python
+      python: '3.6'
       install:
-        - travis_retry pip3 install flake8 --user
+        - travis_retry pip install flake8
       before_script:
         - git fetch --unshallow
       script: