travis : removed external directory from the list under coverage

.travis.yml

@@ -24,7 +24,7 @@ script:
   - spack config get compilers
   - spack install -v libdwarf
   # Run unit tests with code coverage
-  - coverage run --source=lib --omit=lib/spack/spack/test/*,lib/spack/env/*,lib/spack/docs/* bin/spack test
+  - coverage run --source=lib --omit=lib/spack/spack/test/*,lib/spack/env/*,lib/spack/docs/*,lib/spack/external/* bin/spack test
   # Checks if the file that have been changed are flake8 conformant
   - CHANGED_PYTHON_FILES=`git diff develop... --name-only | perl -ne 'print if /\.py/g'`
   - if [[ ${CHANGED_PYTHON_FILES} ]] ; then flake8 --format pylint --config flake8.ini ${CHANGED_PYTHON_FILES} ; fi