drop geckodriver installation

.travis.yml

@@ -1,7 +1,5 @@
 language: python
 cache: pip
-addons:
-  firefox: "45.4.0esr"
 os:
   - linux
 python:
@@ -10,11 +8,6 @@ install:
   - pip install -r requirements.txt
   - pip install flake8
 before_script:
-  # download and extract geckodrive to /usr/local/bin
-  - wget https://github.com/mozilla/geckodriver/releases/download/v0.23.0/geckodriver-v0.23.0-linux64.tar.gz
-  - mkdir geckodriver
-  - tar -xzf geckodriver-v0.23.0-linux64.tar.gz -C geckodriver
-  - export PATH=$PATH:$PWD/geckodriver  # stop the build if there are Python syntax errors or undefined names
   - flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
   # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
   - flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics