[stemcell] fix linux-generic version for trusty.

After running `bundle exec rake stemcell:build_os_image[ubuntu,trusty,$base_os_image_path]` I've got an following 4 errors in rspec execution. Here is an example:
```
1) Ubuntu 14.04 OS image installed by system_kernel Package "linux-headers-3.13.0-35" should be installed
     Failure/Error: it { should be_installed }
       expected Package "linux-headers-3.13.0-35" to be installed
     # ./spec/os_image/ubuntu_trusty_spec.rb:171:in `block (5 levels) in <top (required)>'
```
Looking closer I've found that there is `linux-generic` installation without any version specified `` ([link to code](https://github.com/cloudfoundry/bosh/blob/master/stemcell_builder/stages/system_kernel/apply.sh#L22)). This command installs new version of `linux-headers-3.13.0-36` and `linux-headers-3.13.0-36`. This problem could be solved by adding `.deb` packages to the repo (like is done for `lucid` version).