Travis CI: Fix build validation issues, add Xlint options (TheAlgorithms#2047)

* Travis CI: Fix build validation issues, add Xlint options

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

Author: cclauss

.travis.yml

@@ -1,9 +1,10 @@
+os: linux
+dist: focal
 language: java
 script:
   - find . -type f -name "*.java" > sources.txt
-  - javac @sources.txt
-  
+  - javac -Xlint:deprecation -Xlint:unchecked @sources.txt
 notifications:
     webhooks: https://www.travisbuddy.com/
     on_success: never
-    on_failure: always
+    on_failure: always

DIRECTORY.md

@@ -190,6 +190,9 @@
   * [QueueUsingTwoStacks](https://github.com/TheAlgorithms/Java/blob/master/Others/QueueUsingTwoStacks.java)
   * [RabinKarp](https://github.com/TheAlgorithms/Java/blob/master/Others/RabinKarp.java)
   * [RemoveDuplicateFromString](https://github.com/TheAlgorithms/Java/blob/master/Others/RemoveDuplicateFromString.java)
+  * RestrictedTowerOfHanoi
+    * Main
+      * [Hanoi](https://github.com/TheAlgorithms/Java/blob/master/Others/RestrictedTowerOfHanoi/Main/Hanoi.java)
   * [ReturnSubsequence](https://github.com/TheAlgorithms/Java/blob/master/Others/ReturnSubsequence.java)
   * [ReverseStackUsingRecursion](https://github.com/TheAlgorithms/Java/blob/master/Others/ReverseStackUsingRecursion.java)
   * [RootPrecision](https://github.com/TheAlgorithms/Java/blob/master/Others/RootPrecision.java)