travis-ci: unset compiler for jobs that do not need one

TravisCI does not need to setup any compiler for the documentation
build. Clear the value to fix this.

The Linux32 build job does not define the compiler but it inherits the
value from the base job. Since it does not need the compiler either
because the build runs inside a Docker container we should clear this,
too.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>

.travis.yml

@@ -41,6 +41,7 @@ matrix:
   include:
     - env: Linux32
       os: linux
+      compiler:
       services:
         - docker
       before_install:
@@ -62,7 +63,7 @@ matrix:
         # root@container:/# /usr/src/git/ci/run-linux32-build.sh
     - env: Documentation
       os: linux
-      compiler: clang
+      compiler:
       addons:
         apt:
           packages: