.travis.yml: removing '--use-mirrors' option in 'pip install'
.travis.yml: removing '--use-mirrors' option in 'pip install'

see: https://pip.pypa.io/en/stable/news version 7.0.0
  -> remove of the deprecated [...] --use-mirrors [...] options

urwid/urwid,urwid/urwid,inducer/urwid,tonycpsu/urwid,rndusr/urwid,rndusr/urwid,inducer/urwid,wardi/urwid,wardi/urwid,wardi/urwid,rndusr/urwid,tonycpsu/urwid,inducer/urwid,urwid/urwid,tonycpsu/urwid