travis: Enable tests and add missing dependencies

 - Add doxygen and graphviz for the API doc build.
 - Pass --enable-tests to have the tests built (off by default)

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>

.travis.yml

@@ -4,5 +4,5 @@ compiler:
  - clang
 before_install:
  - sudo apt-get update -qq
- - sudo apt-get install -qq libapparmor-dev libcap-dev libseccomp-dev python3-dev docbook2x libgnutls-dev liblua5.2-dev libselinux1-dev
-script: ./autogen.sh && ./configure && make -j4
+ - sudo apt-get install -qq libapparmor-dev libcap-dev libseccomp-dev python3-dev docbook2x libgnutls-dev liblua5.2-dev libselinux1-dev doxygen graphviz
+script: ./autogen.sh && ./configure --enable-tests && make -j4