Build both static and shared libs on Travis

.travis.yml

@@ -4,8 +4,12 @@ compiler:
   - gcc
   - clang
 
+env:
+  - CMAKE_ARGS="-DBUILD_SHARED_LIBS=OFF"
+  - CMAKE_ARGS="-DBUILD_SHARED_LIBS=ON"
+
 script:
   - mkdir build && cd build
-  - cmake ..
+  - cmake .. ${CMAKE_ARGS}
   - make -j
   - ctest -j