fix build error on MacOS
fix build error on MacOS

This fixes the following build error:

#> grunt test
Running "connect:server" (connect) task
Started connect web server on http://0.0.0.0:9001

Running "jasmine:pivotal" (jasmine) task
Warning: Unable to read
"[..]/Leaflet.Elevation/node_modules/grunt-contrib-jasmine/tasks/lib/../
../node_modules/es5-shim/es5-shim.js" file (Error code: ENOENT). Use
--force to continue.

Aborted due to warnings.

fbonzon/Leaflet.Elevation,MrMufflon/Leaflet.Elevation,MrMufflon/Leaflet.Elevation,MrMufflon/Leaflet.Elevation,fbonzon/Leaflet.Elevation,fbonzon/Leaflet.Elevation