Fix Windows build

This fixes the Windows build failure in Travis CI by:

*Adding an upgrade command for pip, in order to fix the cryptography install error.

*Changing the opencv-python package with the opencv-python-headless version, in order to fix the error in test_bulkresize.py

Author: NikolasMoatsos

.travis.yml

@@ -16,6 +16,7 @@ jobs:
       language: shell
       before_install:
         - choco install python --version 3.7.4
+        - python -m pip install --upgrade pip
       env: PATH=/c/Python37:/c/Python37/Scripts:$PATH
 install:
     - if [ "$TRAVIS_OS_NAME" == "osx" ]; then

installer/requirements.txt

@@ -33,7 +33,7 @@ speedtest-cli
 sympy
 tabulate
 termdown
-opencv-python
+opencv-python-headless
 whois
 wikipedia
 win10toast; sys_platform == 'win32'