Travis-CI update for nix-based build.
Travis-CI update for nix-based build.

(Have to test after the PR...)

alexandergall/snabbswitch,eugeneia/snabb,alexandergall/snabbswitch,eugeneia/snabb,Igalia/snabbswitch,Igalia/snabb,alexandergall/snabbswitch,Igalia/snabb,SnabbCo/snabbswitch,snabbco/snabb,alexandergall/snabbswitch,alexandergall/snabbswitch,snabbco/snabb,alexandergall/snabbswitch,SnabbCo/snabbswitch,Igalia/snabbswitch,snabbco/snabb,eugeneia/snabb,Igalia/snabb,Igalia/snabb,SnabbCo/snabbswitch,Igalia/snabb,Igalia/snabb,snabbco/snabb,eugeneia/snabb,eugeneia/snabb,Igalia/snabbswitch,SnabbCo/snabbswitch,snabbco/snabb,Igalia/snabb,snabbco/snabb,Igalia/snabbswitch,Igalia/snabb,eugeneia/snabb,alexandergall/snabbswitch,alexandergall/snabbswitch,eugeneia/snabb,snabbco/snabb,Igalia/snabbswitch,snabbco/snabb,eugeneia/snabb