Avoid using mk-build-deps if we are newer than Jessie

Currently, build jobs on salsaci suffer from the following problems:

• devscripts, equivs, aptitude and dependencies are installed in the build chroot. Compared to a chroot that only contains Essential, apt and build-essential this means that 114 additional binary packages are installed. This means that it is possible that a salsaci job will not error out if a developer forgot to add a build dependency if it happens to be one of those 114 packages.
• when only building architecture dependent packages in the test-build-any job, only Build-Depends and Build-Depends-Arch must be installed and Build-Conflicts-Indep must be ignored. When only building architecture independent packages in the test-build-all job, only Build-Depends and Build-Depends-Indep must be installed and Build-Conflicts-Arch must be ignored. This is currently not done and cannot be fixed in salsa unless bug #980058 gets fixed for mk-build-deps. This leads to similar false-positive builds like the last point and wrong build dependency declarations will remain unnoticed until the source package is built on the buildds.
• also similarly to the last point, salsaci will install Build-Depends-Indep when doing cross builds. This leads to failing cross builds for packages like gammu which moved a build dependency to build the architecture independent gammu-doc package to Build-Depends-Indep.
• when building packages for experimental, the official buildds use aspcud as the dependency resolver. Since salsaci is not doing the same, this will lead to different behaviour compared to the buildds.

All of the above is fixed by this merge request. This MR replaces the mk-build-deps script by a simple call to apt-get build-dep with the right option depending on the desired package build.

Since apt in Jessie is too old for --indep-only, the new improved behavior is only enabled for releases after Jessie.

This merge request was only made possible with the very useful input from @umlaeute

Fixes: #198 (closed)