travis: shut the fuck up

Travis is currently having "problems" and is spamming IRC all the time
with pointless failure notifications. Make it so that it hopefully sends
a message only when it goes from success to failure, which should
exclude these cases.

While I'm at it, I'd like to complain how idiotic it is to store CI
configuration in a project's source repository. Such a repository should
only contain things that are inherently part of the code's function, not
part of the organization. You don't store bug reports, build results,
the website, developer access controls, etc. in this repository either.
But for CI it's supposed to be OK? I'm all for this "configuration as
code" thing, but what it should mean is that you store configuration
files in some git managed repository, NOT necessarily that you dump them
into the main source code repo. There are many arguments why it should
not be there, such as this very commit. I have a feeling this is mostly
just because all these idiotic web services just want to advertise their
shit and bind customers by not giving easy ways to treat source code and
CI service configuration orthogonal. And so, the source code repo gets
clobbered with stupid shit (both in the set of files and the commit
history). It's fairly idiotic and my tolerance for it is waning. (Oh, of
course you could probably jump through hoops to make it a separate repo,
but I bet that is complicated and has all kinds of downsides because it
won't be the way "it's meant to be used".)

.travis.yml

@@ -107,7 +107,7 @@ notifications:
     channels:
       - "irc.freenode.org#mpv-devel"
     on_success: change
-    on_failure: always
+    on_failure: change
 
 addons:
   coverity_scan: