Fix Travis CI slow build on Project Euler Solution job (#3262)

* Fix Travis CI long run for validate_solutions

* updating DIRECTORY.md

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

Author: dhruvmanila

Diff for: .travis.yml

@@ -16,6 +16,8 @@ jobs:
       script:
         - pytest --doctest-modules --durations=10 --cov-report=term-missing:skip-covered --cov=project_euler/ project_euler/
     - name: Project Euler Solution
+      install:
+        - pip install pytest
       script:
         - pytest --tb=short --durations=10 project_euler/validate_solutions.py
 after_success:

Diff for: DIRECTORY.md

@@ -648,6 +648,8 @@
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_48/sol1.py)
   * Problem 49
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_49/sol1.py)
+  * Problem 51
+    * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_51/sol1.py)
   * Problem 52
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_52/sol1.py)
   * Problem 53