Update bower.json

Included the "main" property as bower is moaning about it's absence.

see -> https://github.com/bower/bower.json-spec#main
Merge pull request #256 from synthomat/patch-1

Update bower.json
Merge branch 'master' into develop

Conflicts:
	bower.json
Updated main field of bower.json to `["dist/vis.min.js", "dist/vis.min.css"]` (see also #256)