TST: Set up Travis-CI to auto build pandas' PRs

[jtratner]

@wesm - can you do this - http://about.travis-ci.org/blog/2012-09-04-pull-requests-just-got-even-more-awesome/ ?

Looks like you just have to tweak a setting and then Travis will build every PR automatically. I don't have admin rights to pydata so I can't change it.

And then we wouldn't have to tell everyone to set up Travis. That would be great

@jtratner, I think you've possibly misread that blog post. Building PRs automatically in their sense
is what you're used to seeing here already.

Since all travis accounts are the same, try to find that option in your own account, I might be wrong,
but I don't think there is one.

An interesting FR for the travis team though, open an issue...

[jtratner]

Not sure, someone from matplotlib said that Travis builds PR's that are
submitted even without the submitter doing anything.

#5378
#1981

[wesm]

@jtratner I just spent about 5 minutes trying to figure out what exactly must be done for this and I could not figure it out. If someone gives me precise instructions about what to do I will do it. Thanks

[jtratner]

Super weird - i'll send them an email and ask. They make it sound like it
would be simple.

[jtratner]

Emailed the Travis people and they set it up. Apparently with old repos you have to enable some other setting. it should start building PR's from now on (even if submitter doesn't have Travis).

So now hopefully it will work.