try using dds in unit test instead of png

Tests/FinalFantasy7/src/test_FF7FLevelFileSerializer.cpp

@@ -139,7 +139,7 @@ BOOST_AUTO_TEST_CASE( read_file )
     BOOST_CHECK_EQUAL( 12345, camera_matrix->getFocalLength() );
 
     Ogre::Image *image( background->createImage( palette ) );
-    image->save( file.getName() + ".png" );
+    image->save( file.getName() + ".dds" ); // Use dds as png codec can't be found during the travis build
     delete image;
 
     QGears::FLevelFilePtr lzs_file = fmgr.load( "reference_compressed.flevel", "General" ).staticCast<QGears::FLevelFile>();
@@ -147,7 +147,7 @@ BOOST_AUTO_TEST_CASE( read_file )
     BOOST_CHECK( !lzs_file->getBackground().isNull() );
 
     image = lzs_file->getBackground()->createImage( lzs_file->getPalette() );
-    image->save( lzs_file->getName() + ".png" );
+    image->save( lzs_file->getName() + ".dds" );
     delete image;
 
     lzs_file.setNull();