Revert "gtksourceview3-no-gi: rename gtksourceview3-no-gi/ext/gtksourceview3/ -> gtksourceview3-no-gi/ext/gtksourceview3-no-gi/"

This reverts commit 8048c6f38e71826863aeefdd2776ccb247399de3.

Because it's strange solution to avoid build error on Travis CI.

With this change, Travis CI says the following:

    extconf.rb: Entering directory `gtksourceview3-no-gi'

    /home/travis/.rvm/rubies/ruby-1.9.3-p551/bin/ruby: No such file or directory -- /home/travis/build/ruby-gnome2/ruby-gnome2/gtksourceview3-no-gi/ext/gtksourceview3/extconf.rb (LoadError)

It's not a good solution. It's just a workaround.