Fix branch for travis badge in readme

[Mark-H]

What does it do

The travis badge shows the build status. This was set to the develop branch, but after changing the branching strategy, this needs to be updated for the 2.x branch.

Why is it needed

Showing the accurate build status.

Related issue(s)/PR(s)

None, though while merging #12801 I noticed it was set wrong.

[pixelchutes]

✗ Job #309.3 - modxcms/revolution - Travis CI

$ phpunit -c ./phpunit.xml
This version of PHPUnit requires PHP 5.6; using the latest version of PHP is highly recommended.

The command "phpunit -c ./phpunit.xml" exited with 1.

Thoughts?

[pixelchutes]

Failing on PHP 5.5 build only.

[Mark-H]

Yeah noticed that, looks like this may contain a solution:
http://andreas.heigl.org/2015/11/12/testing-code-with-phpunit-on-travis-ci-for-php-5-5-and-php7/
Op 5 dec. 2015 6:21 p.m. schreef "Mike Reid" notifications@github.com:

Failing on PHP 5.5 build only.

[image: modxcms_revolution_-_travis_ci]
https://cloud.githubusercontent.com/assets/352182/11609099/f49f6806-9b39-11e5-96a7-580aeb76ea5d.png

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#12803 (comment).

[Mark-H]

Thanks for the quick merge by the way :)

[Mark-H]

Pull sent to fix travis: #12804