[travis] OSX: Force sip version 4.17 due to build problems

.travis.yml

@@ -1,7 +1,5 @@
 matrix:
   fast_finish: true
-  allow_failures:
-    - os: osx
   include:
     # QT4 based build with Python 2.7 // using container based builds and prebuild binary dependencies in osgeo4travis
     - os: linux

ci/travis/osx/before_install.sh

@@ -1,6 +1,8 @@
 brew tap osgeo/osgeo4mac
 brew update
 brew install osgeo/osgeo4mac/qgis-28 --without-postgis --without-postgresql --without-grass --without-gpsbabel --only-dependencies
+brew install https://raw.githubusercontent.com/opengisch/homebrew-versions/sip-4.17/sip417.rb
+brew link --overwrite sip417
 brew install qca
 brew install spawn-fcgi
 brew install lighttpd