Exception when there is no minor version

[ken-duck]

I was running ahab against a list of packages, and it threw an exception whenever there was a package without a minor version. For example:

ca-certificates/now 20170717~16.04.2 all [installed,local]
libkmod2/now 22-1ubuntu5.2 amd64 [installed,local]
libsystemd0/now 229-4ubuntu21.22 amd64 [installed,local]
libudev1/now 229-4ubuntu21.22 amd64 [installed,local]
systemd/now 229-4ubuntu21.22 amd64 [installed,local]
systemd-sysv/now 229-4ubuntu21.22 amd64 [installed,local]
tzdata/now 2019b-0ubuntu0.16.04 all [installed,local]
usbutils/now 1:007-4 amd64 [installed,local]

cc @bhamail / @DarthHater / @ken-duck

[bhamail]

If you have the cycles, could you give this PR a spin and see if it fixes the problem?
#5
If so, I may just merge that puppy.

[DarthHater]

This might be fixed now that #5 hit master, wanna take it for a test spin @ken-duck ?