Try to tackle Travis CI failures

.travis.yml

@@ -5,15 +5,8 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
-before_install:
-  - sudo apt-get update -qq
-  - sudo apt-get install --yes bzr bzrtools git mercurial
 install:
-  - pip install pip-accel
-  - pip-accel install coveralls
-  - pip-accel install --requirement=requirements-checks.txt
-  - pip-accel install --requirement=requirements-tests.txt
-  - LC_ALL=C pip-accel install .
+  - scripts/install-on-travis.sh
 script:
   - make check
   - make test

scripts/install-on-travis.sh

@@ -0,0 +1,25 @@
+#!/bin/bash -e
+
+# Make sure apt-get's package lists are up-to-date.
+sudo apt-get update -qq
+
+# Make sure git is installed.
+sudo apt-get install --yes git
+
+# Install up-to-date versions of Bazaar and Mercurial
+# (using Python packaging instead of Debian packaging).
+if python -c 'import sys; sys.exit(1 if sys.version_info[0] == 2 else 0)'; then
+  sudo apt-get install --yes python-pip
+  sudo pip install --upgrade bzr mercurial
+fi
+
+# Install the required Python packages.
+pip install pip-accel
+pip-accel install coveralls
+pip-accel install --requirement=requirements.txt
+pip-accel install --requirement=requirements-checks.txt
+pip-accel install --requirement=requirements-tests.txt
+
+# Install the project itself, making sure that potential character encoding
+# and/or decoding errors in the setup script are caught as soon as possible.
+LC_ALL=C pip-accel install .