Quote all strings to allow for whitespace.

This is not strictly necessary for all of the strings, but it is generally a
good practice in Bash. In particular, $GOPATH could contain spaces.
Merge branch 'quote-strings' of github.com:juliusv/johnny-deps

Conflicts:
	bin/johnny_deps
pointless checking
exit from main body not function
use PE instead of... that
also fail when vars are unset (eg. $GOPATH)
we already have set -e