Fix build envionment error due to changes in jinja/xapian versions.

3.8 and newer are failing to build with xapian held at 1.8.5 with:

   Could not import extension sphinx.builders.latex
   (exception: cannot import name 'contextfunction' from 'jinja2'

Update to newest xapian on newer platforms.

Author: rouilj

.travis.yml

@@ -51,8 +51,11 @@ addons:
       - gpgsm
 
 before_install:
-  # Sphinx required to build the xapian python bindings
-  - pip install sphinx==1.8.5
+  # Sphinx required to build the xapian python bindings. Use 1.8.5 on
+  # older python and newest on newer.
+  - if [[ $TRAVIS_PYTHON_VERSION == "2."* ]]; then pip install sphinx==1.8.5; fi
+  - if [[ $TRAVIS_PYTHON_VERSION == "3.[67]" ]]; then pip install sphinx==1.8.5; fi
+  - if [[ $TRAVIS_PYTHON_VERSION' != "3.[67]" ]] && [[ $TRAVIS_PYTHON_VERSION == '3.'* ]] ; then pip install sphinx; fi
   - XAPIAN_VER=$(dpkg -l libxapian-dev | tail -n 1 | awk '{print $3}' | cut -d '-' -f 1)
   - cd /tmp
   - curl -s -O https://oligarchy.co.uk/xapian/$XAPIAN_VER/xapian-bindings-$XAPIAN_VER.tar.xz