[fix] Makefile target gh-pages & flatten history of branch gh.pages

1. This patch fixes error:

    rm -rf gh-pages/
    make V=1 gh-pages
    make[1]: Leaving directory '/800GBPCIex4/share/searx'
    [ -d "gh-pages/.git" ] || git clone  gh-pages
    fatal: repository 'gh-pages' does not exist

2. The gh-page build has been moved to ./build/gh-pages this also affects
   'travis-gh-pages'

3. The gh-pages commit messages now includes a ref to the repository and commit

4. Since a gh-pages history has only the drawback that the reposetory grows
   fast, this patch also flattens the history:

    cd build/gh-pages/; git log --oneline
    bash: cd: build/gh-pages/: Datei oder Verzeichnis nicht gefunden
    026126be (HEAD -> gh-pages, origin/gh-pages) make gh-pages: from https://github.com/return42/searx.git@71d66979c2935312e0aed7fc7c3cf6199fbe88a2



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>