Tests, Travis: remove multisite from build matrix.

`grunt tests` or `travis` executes tests for both multisite and regular flavours. As such, we
do not need to configure Travis-CI's build matrix with multisite options.

This change is needed as result of the trunk re-organisation caused by the adoption of Grunt. See #5160

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8382 cdf35c40-ae34-48e0-9cc9-0c9da1808c22