chore: update ubuntu version for ci actions (#79)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index dbf462b..5dda168 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,7 +14,7 @@
 
 jobs:
   build:
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Install JDK 8
         uses: actions/setup-java@v2