Remove renderer for unknown activities

[corinagum]

Fix #1827

This fix removes the render for unknown activities to Web Chat (all activities excluding Message, Event, and Typing). For users wanting to create their own rendering component based off of an activity, take a look at incoming activities and creating custom components sample.

The above screencap is what is rendered when the following activity is sent by the bot:

{
     type: 'x-unknown-activity',
     attachments: [{
       contentType: 'x-unknown-attachment',
       content: {}
     }]
}

[cwhitten]

it looks like the build failed on travis.

[corinagum]

:( On it

[coveralls]

Coverage increased (+0.1%) to 53.199% when pulling 8747a4f on corinagum:1827 into c7a39e6 on Microsoft:master.