Update CI config (#316)

Author: wojtekmach

.github/workflows/ci.yml

@@ -32,7 +32,7 @@ jobs:
 
       - run: mix deps.get && mix deps.unlock --check-unused
         if: ${{ matrix.lint }}
-      
+
       - run: mix deps.compile
 
       - run: mix compile --warnings-as-errors
@@ -62,11 +62,7 @@ jobs:
       - name: Install MySQL Client
         run: |
           apt-get update
-          apt-get install -y wget ca-certificates gnupg
-          apt-key adv --recv-keys --keyserver keys.gnupg.net 8C718D3B5072E1F5
-          echo "deb http://repo.mysql.com/apt/debian/ stretch mysql-${{ matrix.mysql_version }}" >> /etc/apt/sources.list.d/mysql.list
-          apt-get update
-          apt-get install --allow-unauthenticated -y mysql-client
+          apt-get install -y default-mysql-client
           mysql --version
       - uses: actions/checkout@v1
       - name: Install Dependencies
@@ -78,7 +74,7 @@ jobs:
       - run: MYSQL_URL=root:root@mysql ECTO_ADAPTER=myxql mix test
 
   test-mssql:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-16.04
 
     services:
       mssql: