Add six minimum requirement to explicitly resolve conflict

Added pip freeze to travis to ease debugging

.travis.yml

@@ -18,10 +18,12 @@ os: linux
 
 install:
 # Python
+- pip freeze
 - pip install -r monkey/monkey_island/requirements.txt  # for unit tests
 - pip install flake8 pytest dlint  # for next stages
 - pip install coverage  # for code coverage
 - pip install -r monkey/infection_monkey/requirements.txt  # for unit tests
+- pip freeze
 
 # node + npm + eslint
 - node --version

monkey/monkey_island/requirements.txt

@@ -1,7 +1,7 @@
 Flask-JWT>=0.3.2
 Flask-Pymongo>=2.3.0
 Flask-Restful>=0.3.8
-PyInstaller
+PyInstaller==3.6
 awscli>=1.18
 boto3>=1.14
 botocore>=1.17.18,<1.18.0
@@ -19,6 +19,7 @@ python-dateutil>=2.1,<3.0.0
 requests>=2.24
 ring>=0.7.3
 stix2>=2.0.2
+six>=1.13.0
 tornado>=6.0.4
 tqdm>=4.47
 virtualenv>=20.0.26