We need makeinfo >= 5.0 to build on the ci server

Since the travis ci server runs the old Ubuntu LTS it does't have a new makeinfo. Since my documentation simplifications we now require makeinfo 5.0. So we have to manually install makeinfo on the build server for each build. This will no longer necessary once Travis upgrades to the new Ubuntu LTS, i.e. 14.04 (see also travis-ci/travis-ci#2046).