migrate fpb-lint from travisci to gh actions (#5186)

.github/workflows/fpb-lint.yml

+name: free-programming-books-lint
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Use Node.js
+      uses: actions/setup-node@v1
+      with:
+        node-version: '6.x'
+    - run: npm install -g free-programming-books-lint
+    - run: fpb-lint .

.travis.yml

-language: shell
-dist: xenial
-os: linux
-
-jobs:
-  include:
-    -
-      language: node_js
-      node_js:
-        - 6
-      before_script:
-        - npm install -g free-programming-books-lint
-      script:
-        - fpb-lint .