chore(.travis.yml): use git clean to clean out untracked files

PR #641 involved renaming a directory. The old directory was still
present in the cache, and in this situation `git status` lists the
directory as a whole as untracked, so the grep did not find any
`.lean` files.

.travis.yml

@@ -20,8 +20,7 @@ install:
   - chmod +x $HOME/scripts/travis_long
   - (git status | grep  -e "Changes not staged for commit:"); RESULT=$?
   - if [ $RESULT -eq 0 ]; then git checkout -f HEAD ; fi
-  - rm `git status | grep "\.lean" | sed "s/\.lean/.olean/"` ||  true
-  - rm `git status | grep "\.lean"` || true
+  - git clean -d -f -q
   - rm mathlib.txt || true
 
 jobs: