Make .travis.yml be valid

[eksperimental]

Before:

$ travis lint .travis.yml
Warnings for .travis.yml:
[x] value for notifications section is empty, dropping
[x] in notifications section: unexpected key recipients, dropping

Now:

$ travis lint .travis.yml
Hooray, .travis.yml looks valid :)

[josevalim]

If it was not used and I was receiveing e-mauls, then let's just remove it. They probably send to everyone who is a maintainer by default.

[whatyouhide]

@josevalim they do :)