travis-ci: enable Git SVN tests t91xx on Linux

Install the "git-svn" package to make the Perl SVN libraries available
to the Git SVN tests on Travis-CI Linux build machines.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Eric Wong <e@80x24.org>

.travis.yml

@@ -18,6 +18,7 @@ addons:
   apt:
     packages:
     - language-pack-is
+    - git-svn
 
 env:
   global: