Revert "Remove Java 15-ea"

This reverts commit c6ccfa9.

Author: bitwiseman

.github/workflows/maven-build.yml

@@ -51,7 +51,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu, windows ]
-        java: [ 8, 11, 13, 14 ]
+        java: [ 8, 11, 13, 15-ea ]
     steps:
     - uses: actions/checkout@v2
     - name: Set up JDK