Leaflet performance issues in IE11 and Edge

[mruettgers]

In IE11 and Edge the map initialization is very, very slow (for versions < 0.7.4 and >= 0.7.7 (due to a regression)).
Sticking leaflet to 0.7.5 solves the issue here.

Leaflet/Leaflet#4041
Leaflet/Leaflet#4061

[tcatm]

So we should wait for a fixed leaflet release before releasing meshviewer v5.