No longer test on Travis.

We are tested on jenkins.plone.org, and the Travis setup on master is pretty broken.

The master branch has been failing for a long time with this error:

An error occurred while generating the build script.

https://lint.travis-ci.org was complaining about the addons section.
This can be tricky to get right, because the linter errors are not entirely obvious:
- value for addons section is empty, dropping
- in addons section: dropping sauce_connect section: unexpected sequence

In our case, we had dashes in front of the options, and that broke it.

But after fixing that, it was still very broken, missing distributions.

CHANGES.rst

@@ -14,6 +14,9 @@ New features:
 
 Bug fixes:
 
+- No longer test on Travis.  We are tested on jenkins.plone.org, and
+  the Travis setup on master is pretty broken.  [maurits]
+
 - Take more time during robot tests.
   I hope that this makes a sometimes failing test always pass.  [maurits]
 

Makefile

@@ -3,9 +3,6 @@ NIX_PATH=~/.nix-defexpr/channels/
 all: bootstrap
 	bin/buildout -v
 
-travis: bootstrap
-	bin/buildout -c travis.cfg -v
-
 bootstrap:
 	NIX_PATH=${NIX_PATH} nix-build --out-link nixenv dev.nix
 	./nixenv/bin/virtualenv --distribute --clear .