Update travis build to verify all templates

[sayedihashimi]

We already have a travis build in place. When there are changes it's cumbersome to verify. It would be good if travis could verify the following continue to work.

• yo aspnet creates content for all generators
• kpm restore passes for all generators
• kpm build passes for all generators
• k kestrel successfully starts kestrel on given port for web projects. _stretch goal_

I don't know much about travis myself so this is def an area that we can use some help.

[spboyer]

@sayedihashimi PR #59 verifies that all yo aspnet and yo aspnet:[generator] validates. I also added the Travis CI hook, you can see here https://travis-ci.org/OmniSharp/generator-aspnet/builds/49649866 when it fails for example on one of the PR test I put in.