[Travis] try setting install rpath as a cache arg

CMakeExternals/DCMTK.cmake

@@ -56,11 +56,11 @@ if(NOT DEFINED DCMTK_DIR AND NOT ${CMAKE_PROJECT_NAME}_USE_SYSTEM_${proj})
     CMAKE_ARGS
       -DDCMTK_INSTALL_BINDIR:STRING=bin/${CMAKE_CFG_INTDIR}
       -DDCMTK_INSTALL_LIBDIR:STRING=lib/${CMAKE_CFG_INTDIR}
-      -DCMAKE_INSTALL_RPATH:STRING=${ep_install_dir}/lib
     CMAKE_CACHE_ARGS
       ${ep_common_cache_args}
       ${ep_project_include_arg}
       -DBUILD_SHARED_LIBS:BOOL=ON
+      -DCMAKE_INSTALL_RPATH:STRING=${ep_install_dir}/lib
       -DDCMTK_WITH_DOXYGEN:BOOL=OFF
       -DDCMTK_WITH_ZLIB:BOOL=OFF # see github issue #25
       -DDCMTK_WITH_OPENSSL:BOOL=OFF # see github issue #25