Travis-CI: also build the skeleton plugin

The skeleton plugin is an example and as such needs to work out-of-the-box.
Make travis build it to ensure we don't break anything.

Signed-off-by: David Wagner <david.wagner@intel.com>
diff --git a/.travis.yml b/.travis.yml
index 12f177a..d5e8b75 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,7 +9,11 @@
   - sudo apt-get install -y swig
 
 # how to build
-script: cmake . && make -j && sudo make install && make test
+script:
+  - cmake . && make -j && sudo make install
+  - make test
+  - cd skeleton-subsystem
+  - cmake . && make && sudo make install
 
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