Re-add basic TravisCI testing

Add basic, fast TravisCI script to keep the TravisCI jobs alive even
if GitHub CI goes down.  Belt-and-braces, but it's free.

Limit the runs on the Travis jobs to 5 short ones in total to match
the runtime of the GitHubCI.

Author: earlephilhower

.travis.yml

@@ -0,0 +1,70 @@
+# TravisCI left in repo as a backup CI solution in case GitHub CI suffers a
+# major disruption.  Only a few, quick tests are run in order to keep the
+# TravisCI runtime equal to the GitHub CI time (to avoid bottlenecking on
+# TravisCI).
+#
+# If GitHub CI goes away, it would make sense to drop this .YML file and
+# use the complete one from release tag 2.7.2
+
+
+language: bash
+os: linux
+dist: bionic
+
+git:
+  depth: 1
+  submodules: false
+
+before_install:
+  - git submodule update --init  # no recursive update
+
+stages:
+  - build
+  - deploy
+
+
+# Run only 5 jobs since TravisCI only allows 5 in parallel
+jobs:
+  include:
+    - name: "Platform.IO"
+      stage: build
+      script: $TRAVIS_BUILD_DIR/tests/platformio.sh
+      install:
+          - sudo apt-get install python3-pip python3-setuptools
+      env:
+        - BUILD_PARITY=custom mod=10 rem=1
+
+    - name: "Build 1/4"
+      stage: build
+      script: $TRAVIS_BUILD_DIR/tests/build.sh
+      env:
+        - BUILD_PARITY=custom mod=10 rem=1
+
+    - name: "Build 2/4"
+      stage: build
+      script: $TRAVIS_BUILD_DIR/tests/build.sh
+      env:
+        - BUILD_PARITY=custom mod=10 rem=2
+
+    - name: "Build 3/4"
+      stage: build
+      script: $TRAVIS_BUILD_DIR/tests/build.sh
+      env:
+        - BUILD_PARITY=custom mod=10 rem=3
+
+    - name: "Build 4/4"
+      stage: build
+      script: $TRAVIS_BUILD_DIR/tests/build.sh
+      env:
+        - BUILD_PARITY=custom mod=10 rem=4
+
+notifications:
+  email:
+    on_success: change
+    on_failure: change
+  webhooks:
+    urls:
+      - secure: "dnSY+KA7NK+KD+Z71copmANDUsyVePrZ0iXvXxmqMEQv+lp3j2Z87G5pHn7j0WNcNZrejJqOdbElJ9Q4QESRaAYxTR7cA6ameJeEKHiFJrQtN/4abvoXb9E1CxpL8aNON/xgnqCk+fycOK3nbWWXlJBodzBm7KN64vrcHO7et+M="
+    on_success: change  # options: [always|never|change] default: always
+    on_failure: always  # options: [always|never|change] default: always
+    on_start: false     # default: false