Travis CI: Use flake8 to find Python syntax errors

Also, put the execution steps in order:
1. before_install -->
2. install -->
3. before_script -->
4. script -->
5. after success -->
6. notifications

.travis.yml

@@ -7,8 +7,13 @@ python:
 dist: xenial
 services:
  - docker
+cache: pip
+before_install:
+  - pip install --upgrade pip wheel
+  - pip install codecov coveralls flake8 mock pytest pytest-cov selenium
+  # - docker build -t zeronet .
+  # - docker run -d -v $PWD:/root/data -p 15441:15441 -p 127.0.0.1:43110:43110 zeronet
 install:
- - pip install -U pip wheel
  - pip install -r requirements.txt
  - pip list
 before_script:
@@ -26,18 +31,10 @@ script:
  - python -m pytest src/Test --cov=src --cov-config src/Test/coverage.ini
  - mv plugins/disabled-Multiuser plugins/Multiuser && python -m pytest -x plugins/Multiuser/Test
  - mv plugins/disabled-Bootstrapper plugins/Bootstrapper && python -m pytest -x plugins/Bootstrapper/Test
-before_install:
-  - pip install -U pytest mock pytest-cov selenium
-  - pip install codecov
-  - pip install coveralls
-  # - docker build -t zeronet .
-  # - docker run -d -v $PWD:/root/data -p 15441:15441 -p 127.0.0.1:43110:43110 zeronet
+ - flake8 . --count --select=E9,F63,F72,F82 --show-source --statistics
 after_success:
   - codecov
   - coveralls --rcfile=src/Test/coverage.ini
-cache:
-  directories:
-    - $HOME/.cache/pip
 notifications:
   email:
     recipients: