[Bugfix] Fix for #974

[pgierz]

Hi, I have a quick fix for #974; we simply check if a file /etc/os-release exists, and grep for the appropriate release id if it does. If it does not the case statement reverts back to the default linux icon. This isn't so nice if you do in face have one of the pre-made icons (e.g. centos), but if the files are named inconsistently between different hosts, I don't see a good way of getting around that.

[bhilburn]

@pgierz - Great idea! The only thing I want to change about this PR is the use of grep - we are trying to remove as many calls to external programs as possible in the next codebase.

@onaforeignshore @rjorgenson - Any chance you can suggest a replacement pure-ZSH alternative to the use of grep, here? =)

[bhilburn]

Okay, I just realized that #988 fixed this in the next branch, hah. So, the issue is fixed in next, but not in master. Also, the fix indeed still uses grep.

If we are able to replace the use of grep in this PR, I'd like to merge this into master and then replace the same code in next with this change, as well.

[dritter]

Hmm. Once again the travis tests on OSX hangs. Merging anyway. Thx @pgierz .