Fixed one test that didn't use the configured root URL

There was a hardcoded 'http://localhost', if you use a different port that 80 then the test fails
In generate:travis-yml command, look for tests in root tests folder as well as subfolders.
fixes #5958 When there is an error screen in Piwik (core or in Tracker) then log to error log
Refs #5962, add ability for .travis.yml file to check out dependent plugins using DEPENDENT_PLUGINS travis environment variable (set in .travis.yml file manually).
Send git cloning output to /dev/null in checkout_dependent_plugins.sh travis script.
fixes #5415 Introducing `storeCustomVariablesInCookie` function.
Merge branch 'master' of github.com:piwik/piwik
Do not redirect diff output to /dev/null to aid in debugging strange failures (in autoupdate travis yml script).
refs #5415 fix tests, call storeCustomVariablesInCookie
2.5.0-rc1
Make new script executable.
Better debug output for forum debugging http://forum.piwik.org/read.php?2,117670
Make sure statement to update travis scripts is always executed before script running is attempted.
.travis.yml file is out of date, auto-updating .travis.yml file.
Add more debugging information to checkout_dependent_plugins.sh script.
Fix for last commit.
Fixing plugin name detection in checkout_dependent_plugin.sh.
Make sure git checkout master does not fail on other branches when running in travis.
.travis.yml file is out of date, auto-updating .travis.yml file.
Fix git checkout command in travis.yml generator and correctly reference self:: instead of -> in IntegrationTestCase.php.