Update copyright in LICENSE.txt (#2067)

* Update LICENSE.txt

* add link to contributors

LICENSE.txt

@@ -1,4 +1,6 @@
-Copyright (C) 2013, Rob Story
+Copyright (C) 2013-, Folium developers
+See https://github.com/python-visualization/folium/graphs/contributors for a
+full list of contributors.
 
 Permission is hereby granted, free of charge, to any person obtaining a copy of
 this software and associated documentation files (the "Software"), to deal in
@@ -16,4 +18,4 @@ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.
+SOFTWARE.