TravisCI/Linux: Disable system superlu

Recent commit for Linux enabled by default,
good default for typical Linux installs, but not working for TravisCI.

ci-scripts/linux/travis-build.sh

@@ -4,7 +4,8 @@ CFLAGS="-fPIC" CXXFLAGS="-fPIC" ./configure --disable-jbig && make
 popd
 cd toonz && mkdir build && cd build
 source /opt/qt55/bin/qt55-env.sh
-cmake ../sources
+cmake ../sources \
+    -DWITH_SYSTEM_SUPERLU:BOOL=OFF
 # according to https://docs.travis-ci.com/user/ci-environment/#Virtualization-environments
 # travis can offer up to 2 cores in burst, try using that
 make -j 2