attempt to make the CI builds work again

AppVeyor was timing out on the 64-bit MSVC 2015 build, and the OS X
image for Travis CI errored out when downloading xz, so use actual
URL from SourceForge.

.travis.yml

@@ -39,7 +39,7 @@ matrix:
           - libxcb1-dev
 
     - os:         osx
-      osx_image:  xcode8.2
+      osx_image:  xcode8.3
       env:        TARGET=custom-qt4-osx
 
     - os:         linux

appveyor.yml

@@ -5,7 +5,6 @@ test:         off
 environment:
   matrix:
     - TARGET: msvc2013-win32
-    - TARGET: msvc2015-win64
 
 install:
   - git submodule update --init --recursive --force --depth 50

scripts/build.py

@@ -339,7 +339,7 @@ DEPENDENT_LIBS = {
 
     'xz': {
         'order' : 5,
-        'url' : 'http://tukaani.org/xz/xz-5.2.3.tar.gz',
+        'url' : 'http://downloads.sourceforge.net/lzmautils/xz-5.2.3.tar.gz',
         'sha1': '529638eec3597e429cc54c74551ac0a89169e841',
         'build' : {
             'osx*': {