dropped Python 2.5 support

it appears there are issues with Jinja2:
https://travis-ci.org/FND/tiddlywebplugins.bfw/jobs/7491098

    E File ".../python2.5/site-packages/jinja2/environment.py", line 648
    E info.external_attr = 0o755 << 16
    E ^
    E SyntaxError: invalid syntax