Add "main" field to bower.json
Add "main" field to bower.json

This should fix the following message from `rails-assets-tether`:
```
This component doesn't define main assets in bower.json.
Please open new pull request in component's repository:
https://github.hubspot.com/tether
```
dandv/tether,chrismoulton/tether,darobin/tether,martijnrusschen/tether,StileEducation/tether,xLegoz/tether,HubSpot/tether,lneir/tether,xLegoz/tether,ktardthong/tether,SRobertZ/tether,martijnrusschen/tether,varblob/tether,mcanthony/tether,SRobertZ/tether,dieface/tether,dandv/tether,pzuraq/tether