browser tests: apache needs write access to images/

When uploading files via browser tests, MediaWiki will need write access
to the images/ directory :]

Jobs updated:

mwext-browsertests-CirrusSearch-phantomjs
mwext-browsertests-Flow-firefox
mwext-browsertests-Flow-phantomjs
mwext-browsertests-MobileFrontend-firefox
mwext-browsertests-MobileFrontend-phantomjs
mwext-browsertests-Translatewiki-firefox
mwext-browsertests-Translatewiki-phantomjs
mwext-browsertests-UniversalLanguageSelector-firefox
mwext-browsertests-UniversalLanguageSelector-phantomjs
mwext-browsertests-VisualEditor-firefox
mwext-browsertests-VisualEditor-phantomjs

Change-Id: I1c34bb874342958fafa569e46674f64ed03d8e9b