Replace TravisCI with Github Actions

This change adds the necessary files and logic to take advantage of
GitHub Actions for running our automated tests. Currently we rely on
TravisCI to do this, but we'd like to transition over to GitHub Actions,
for the following reasons:

    1. The Actions UI is more integrated with GitHub PRs
    2. Best practices at Delphix suggests using Actions

Additionally, this change removes integration with the "bors" bot.

Author: Prakash Surya

.github/workflows/main.yml

@@ -0,0 +1,21 @@
+on: [push, pull_request]
+
+jobs:
+  check-packages:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - run: docker build -t delphix-platform:latest docker
+      - run: ./scripts/docker-run.sh make packages
+  check-shellcheck:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - run: docker build -t delphix-platform:latest docker
+      - run: ./scripts/docker-run.sh make shellcheck
+  check-shfmt:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - run: docker build -t delphix-platform:latest docker
+      - run: ./scripts/docker-run.sh make shfmtcheck