Use composer to load Text Diff library used in automatic updater

Currently we have an old modified version of Text Diff being used for our updater. The code was modified primarily so that it meets our coding guidelines.

Since our coding guidelines have changed since then and we can use external libraries, we should remove our forked code and use their library, then update all calls to the library as necessary.

We use the latest version of Text_Diff, but it has since been superseded by Horde_Text_Diff.