[travis] update gnutls bug workaround

.travis.yml

@@ -37,6 +37,7 @@ before_install:
     : ${BUILD_CMD:="ninja -k 10000 -j 2"} && export BUILD_CMD
   - cd $HOME
   # workaround for gnutls bug is using git built against curl-openssl (cannot go into apt script below)
+  - sudo -E rm /etc/apt/sources.list.d/*git*.list
   - sudo -E add-apt-repository -y ppa:pymor/travis && sudo -E aptitude update && sudo -E aptitude install git -yq
   - test -d ${SUPERDIR}/ || git clone https://github.com/dune-community/dune-xt-super.git ${SUPERDIR}/
   - cd ${SUPERDIR}/