Test if FOSS_DEVEL is a specific string, not just that it exists

A bit more specificity here allows us saner packaging repo use.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #7 from MosesMendoza/fix_pbbuilderrc_foss_devel

Test if FOSS_DEVEL is a specific string, not just that it exists