Fix errors in Makefile for linux

This fixes some errors I was seeing when running `make update` under ubuntu.

* `pushd` is a bash builtin so specifying bash was required.
* the second issue was the git pull was complaining about not a branch e.g:

   ```
You are not currently on a branch, so I cannot use any
'branch.<branchname>.merge' in your configuration file.
Please specify which branch you want to merge with on the command
line and try again (e.g. 'git pull <repository> <refspec>').
See git-pull(1) for details.
```

Would be useful if someone on a mac could check this doesn't break anything for them.