workaround 'first run' tox issue

tox installs dependencies first, which includes mwlib.rl. mwlib.rl pulls
in the last released version of mwlib. tox then installs the mwlib it
has build from source without pip's -U flag, that is it won't update the
installed version and we're basically testing against the last released
version.

we work around this issue by setting 'install_command', which requires
a tox version later or equal 1.6.