ensure copyright banner survives minification

[jgravois]

v1.1.0 is missing the copyright banner.

adding @preserve (and --comments in the uglify CLI command) resolves the problem.

prior to making this patch i encountered an unrelated error:

$ uglifyjs --version
uglify-js 3.0.27

$ npm run uglify

> leaflet@1.1.0 uglify /Users/john/github/Leaflet
> uglifyjs dist/leaflet-src.js -c -m -o dist/leaflet.js --source-map dist/leaflet.js.map --in-source-map dist/leaflet-src.js.map --source-map-url leaflet.js.map

Supported options:
  content         null
  filename        null
  includeSources  false
  root            null
  url             null
ERROR: `dist/leaflet.js.map` is not a supported option

substituting filename=dist/leaflet.js.map took care of that as well.

[perliedman]

Thanks! Great catches both of them.