travis-ci: enable web server tests t55xx on Linux

Install the "apache" package to run the Git web server tests on
Travis-CI Linux build machines. The tests are already executed on OS X
build machines since the apache web server is installed by default.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.travis.yml

@@ -18,6 +18,7 @@ addons:
   apt:
     packages:
     - language-pack-is
+    - apache2
 
 env:
   global:
@@ -30,6 +31,7 @@ env:
     - DEFAULT_TEST_TARGET=prove
     - GIT_PROVE_OPTS="--timer --jobs 3 --state=failed,slow,save"
     - GIT_TEST_OPTS="--verbose --tee"
+    - GIT_TEST_HTTPD=true
     - GIT_TEST_CLONE_2GB=YesPlease
     # t9810 occasionally fails on Travis CI OS X
     # t9816 occasionally fails with "TAP out of sequence errors" on Travis CI OS X