fix(CI): add selenium to travis

.travis.yml

@@ -10,7 +10,7 @@ script:
  - python -m pytest plugins/CryptMessage/Test
  - python -m pytest src/Test --cov=src --cov-config src/Test/coverage.ini
 before_install:
-  - pip install -U pytest mock pytest-cov
+  - pip install -U pytest mock pytest-cov selenium
   - pip install codecov
   - pip install coveralls
 after_success:
@@ -18,4 +18,4 @@ after_success:
   - coveralls --rcfile=src/Test/coverage.ini
 cache:
   directories:
-    - $HOME/.cache/pip
+    - $HOME/.cache/pip