Auto-detect image magick latest 6.9.X-Y version (#936)

* Auto-detect image magick version

* Fix version detection

appveyor.yml

@@ -2,7 +2,6 @@ image: Visual Studio 2017
 
 environment:
   global:
-    IMAGE_MAGICK_VERSION: 6.9.10-35-Q16
     IMAGE_MAGICK_INSTALL_DIR: c://ImageMagick
   matrix:
     - PYTHON: "C:\\Python35-x64"
@@ -36,6 +35,12 @@ install:
   - pip install --upgrade pip --user
   - pip install pipenv
 
+  # Find latest image magick version by parsing website
+
+  - python find_latest_imagemagick_version.py > temp.txt
+  - set /p IMAGE_MAGICK_VERSION=<temp.txt
+
+  - echo %IMAGE_MAGICK_VERSION%
   # Download ImageMagick installer (which also installs ffmpeg.)
   # From http://ftp.fifi.org/ImageMagick/binaries/
   # Might need to be updated from time to time

find_latest_imagemagick_version.py

@@ -0,0 +1,21 @@
+from urllib import request
+import re
+
+url = "https://legacy.imagemagick.org/script/index.php"
+
+'''This little script parses url above to extract latest image magick version
+(major version 6.9), to feed it into CI system. Not the best way for reproducible
+builds, but it's preferred for now over storing imagemagick installer into the
+git repository
+'''
+
+response = request.urlopen(url)
+html = response.read().decode('utf-8')
+r = re.compile("6\.9\.[0-9]+\-[0-9]+")
+version = r.findall(html)
+if len(version) == 0:
+    raise ValueError("Could not find latest legacy 6.9.X-Y ImageMagick version from {}".format(url))
+version = version[0]
+# Append Q16 build
+version += '-Q16'
+print(version)