Add a travis test for checking external symbols.

Adds a test which lists symbols in the static library and fails if any
global symbol does not have the kvz_ prefix.

.travis.yml

@@ -18,7 +18,13 @@ matrix:
       env: KVZ_DISABLE_AVX2=""
     - compiler: gcc-4.8
       env: KVZ_DISABLE_AVX2=""
-    
+
+    # Check for external symbols without kvz_ prefix.
+    - script:
+        - cd src
+        - make libkvazaar.a
+        - (! nm -go --defined-only libkvazaar.a | grep -v ' kvz_')
+
     # These valgrind tests are slow, so they are performed with the minimum
     # number of small frames and fast settings.