Switch to github workflows (elixir-editors#420)

* Switch to github workflows

Travis open source version is going away at the end of the year. Also
the wait times are an hour+

* Update versions of elixir/erlang tested

.github/workflows/ci.yml

@@ -0,0 +1,89 @@
+name: CI
+
+on:
+  pull_request:
+    branches:
+      - master
+  push:
+    branches:
+      - master
+
+jobs:
+  mix_test:
+    name: mix test (Elixir ${{matrix.elixir}} | Erlang/OTP ${{matrix.otp}})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - elixir: 1.8.x
+            otp: 21.3.8.18
+            tests_may_fail: false
+          - elixir: 1.9.x
+            otp: 21.3.8.18
+            tests_may_fail: false
+          - elixir: 1.10.x
+            otp: 21.3.8.18
+            tests_may_fail: false
+            check_unused_deps: true
+          - elixir: 1.11.x
+            otp: 21.3.8.18
+            tests_may_fail: false
+            check_unused_deps: true
+          - elixir: 1.11.x
+            otp: 23.1.4
+            tests_may_fail: false
+            warnings_as_errors: true
+            check_formatted: true
+            check_unused_deps: true
+            run_dialyzer: true
+    env:
+      MIX_ENV: test
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-elixir@v1
+        with:
+          otp-version: ${{matrix.otp}}
+          elixir-version: ${{matrix.elixir}}
+      - name: Install Dependencies
+        run: |
+          mix local.hex --force
+          mix local.rebar --force
+          mix deps.get --only test
+      - run: mix format --check-formatted
+        if: matrix.check_formatted
+      - run: mix compile --warnings-as-errors
+        if: matrix.warnings_as_errors
+      - run: mix test || ${{ matrix.tests_may_fail }}
+
+  static_analysis:
+    name: static analysis (Elixir ${{matrix.elixir}} | Erlang/OTP ${{matrix.otp}})
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include:
+          - elixir: 1.11.0
+            otp: 23.1.1
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-elixir@v1
+        with:
+          otp-version: ${{matrix.otp}}
+          elixir-version: ${{matrix.elixir}}
+      - name: Cache build artifacts
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.hex
+            ~/.mix
+            _build
+          key: ${{ matrix.otp }}-${{ matrix.elixir }}-static_analysis-build
+      - name: Install Dependencies
+        run: |
+          mix local.hex --force
+          mix local.rebar --force
+          mix deps.get
+      - name: Restore timestamps to prevent unnecessary recompilation
+        run: IFS=$'\n'; for f in $(git ls-files); do touch -d "$(git log -n 1 --pretty='%cI' -- $f)" "$f"; done
+      - run: mix dialyzer
+        if: matrix.run_dialyzer