fix(runner): skip sourcing if no file exists

Before, slugrunner would try to source .profile.d/* as a file,
which would fail because .profile.d/* is not present. Setting
noglob will skip the loop if no file is present in the directory.

To show the difference between the two commands:

```
$ for file in .profile.d/*; do echo $file; done
.profile.d/*
$ for file in `ls .profile.d`; do echo .profile.d/$file; done
$ shopt -s noglob
$ for file in .profile.d/*; do echo $file; done
$ for file in `ls .profile.d`; do echo .profile.d/$file; done
```

See http://www.gnu.org/software/bash/manual/html_node/The-Shopt-Builtin.html

Signed-off-by: Matthew Fisher <matthewf@opdemand.com>