fix error when scanning folders for feteched update for Alpine vulnerabilities

[alexei-led]

Fix error updater: an error occured when fetching update 'alpine': open /private/tmp/alpine-secdb808866024/Makefile/main.yaml: not a directory.

This error happens on MacOS with go version go1.7.4 darwin/amd64.

For some strange reason File.Readdirnames returns names of directories and files. I've changed this code to use File.Readdir that returns []Fileinfo and thus adding additional validation that skips non-directories, using IsDir() method.

[jzelinskie]

Thanks for the PR! Strange corner-case indeed.

Your commit seems to also make unrelated changes to the vendor directory, though.

[alexei-led]

changes to vendor folder are not related indeed. I just run "glide install" to build and test clair and glide updated couple files in vendor folder. Ignore these, sorry.

[jzelinskie]

If these changes are indeed pulled in by glide install, but are not currently apart of the vendor directory, we should commit them. However, they should be in a separate commit. Likewise if you'd like to add a .gitignore file, please put it in a separate commit. Having individual commits like this helps in the future when you need to use tools like git blame to remember why a specific change was made.

[alexei-led]

OK, fixed this PR to include only one change, that fixes described issue

[jzelinskie]

LGTM. Thanks for adjusting the PR!