chore(ci) purge tree before installing Kong
chore(ci) purge tree before installing Kong

It seems the Luarocks install cached by Travis retains some data about the installed
modules. Better to purge the tree before installing Kong, to avoid
conflicts with module changes bearing the same version. (Sometimes
modules are changed but Kong's version is not bumped until its release)

Kong/kong,Vermeille/kong,Kong/kong,icyxp/kong,shiprabehera/kong,Kong/kong,beauli/kong,li-wl/kong,jebenexer/kong,akh00/kong,ccyphers/kong,salazar/kong,Mashape/kong,smanolache/kong,jerizm/kong