DRYing the code

[aleksandrs-ledovskis]

In the implementation of #63 I have noticed that by cursory look the fetch_pkg_age function is duplicated between brew-safe-upgrade and brew-safe-install scripts.

Unless this is due to some architectural decision, the general convention is to simplify and extract common code blocks to a reusable module which can be included in multiple places.

So, my suggestion is to improve this area a bit - improve the code by DRY'ing and re-checking for conformance to Shell script development best practices and formatting.