Fix EZP-20851: Setup Travis tests to also test prostgres

Branch fixing the field-case-insensitivity issues with PostGreSQL (and Oracle) on top of patrickallaert:EZP-20851 – @docb22 asked me to take a look at this.

The values in the database are already lowercased during indexing using the transformation processor in ezpublish-legacy. Thus the same transformation is now applied during search.

Once we implement indexing in ezpublish-kernel for the legacy backend we MUST make sure to also lowercase the values in sort_key_string.

If you want me to rebase the branch on top of something else, please add a remark. I used @patrickallaert branch as a base, because the PostGreSQL tests actually work in this branch. Thanks for taking care of that. :-) Makes the diff ugly, though.