Install mercurial package on Travis

This allows us to have Mercurial tested on Python 3 as well.
Signed-off-by: Michal Čihař <michal@cihar.com>

.travis.yml

@@ -54,3 +54,7 @@ matrix:
       env: CI_PIP_DEPS="Django>=1.9,<1.10 https://github.com/translate/translate/zipball/master"
     - python: "3.4"
       env: CI_PIP_DEPS="Django>=1.9,<1.10 https://github.com/translate/translate/zipball/master" CI_MODE=lint
+addons:
+  apt:
+    packages:
+    - mercurial