fix processing deb files

[itzunagi]

Fixes a regression in the 2.0.5 release which caused staging to fail whenever an Aptfile was used to specify system dependencies to be installed.

[christiancompton]

I have validated the test cases set forth by @rolivieri, using libpython2.7 in the Aptfile.

Test 1

• Remove any instances of Kitura-Starter from your space (cf delete Kitura-Starter).
• Provision Kitura-Starter (https://github.com/IBM-Bluemix/Kitura-Starter) using v2.0.4 of the buildpack -> cf push -b swift_buildpack_v2_0_4-20170125-2344

• Once Kitura-Starter is provisioned and running, let's do another cf push but this time using the latest code with the changes: cf push -b https://github.com/IBM-Bluemix/Kitura-Starter#

• The application should provision just fine and we should not run into the error the user described here -> https://github.com/IBM-Swift/swift-buildpack/pull/78/files

Test 2

• Add an Aptfile to the Kitura-Starter project in your local file system with the following content:

$ cat Aptfile libmysqlclient-dev

• Provision Kitura-Starter using the latest code with the changes: cf push -b https://github.com/IBM-Bluemix/Kitura-Starter#
• While the application is provisioned, we should see output like this:

-----> Entry found in Aptfile for libmysqlclient-dev. -----> Fetching .debs for: libmysqlclient-dev ...

Using libpython2.7:

• The application should be provisioned without errors.

I am merging this into develop, but will wait a couple of hours for @rolivieri to potentially review before I kick off the deploy-release and YS0 steps/