Update appveyor config (lxml 4.2.5 via pip install)

Author: mvantellingen

appveyor.yml

@@ -12,8 +12,8 @@ environment:
 
 install:
   - "%PYTHON%/Scripts/easy_install -U pip"
-  - "%PYTHON%/Scripts/easy_install lxml==3.5.0"
   - "%PYTHON%/Scripts/pip install wheel"
+  - "%PYTHON%/Scripts/pip install --only-binary :all: lxml==4.2.5"
   - "%PYTHON%/Scripts/pip install -e .[test]"
 
 test_script: