Fix test_eligible_kegs_for_cleanup for Linuxbrew
Fix test_eligible_kegs_for_cleanup for Linuxbrew

Fails on Ubuntu 15 (wily) running on Amazon EC2.

The directories returned by installed_kegs are in the ordered returned
by Pathname::children, which is arbitrary and determined by the
operating system and file system.

Failure:
FormulaTests#test_eligible_kegs_for_cleanup [/home/ubuntu/.linuxbrew/Library/Homebrew/test/test_formula.rb:366]:
--- expected
+++ actual
@@ -1,2 +1,2 @@
 [#<Keg:/tmp/homebrew_tests20160307-25418-1w2960h/cellar/testball/0.2>,
- #<Keg:/tmp/homebrew_tests20160307-25418-1w2960h/cellar/testball/0.3>]
+ #<Keg:/tmp/homebrew_tests20160307-25418-1w2960h/cellar/testball/0.1>]

Closes Linuxbrew/linuxbrew#895

Linuxbrew/brew,maxim-belkin/brew,hanxue/linuxbrew,hanxue/linuxbrew,hanxue/linuxbrew,rwhogg/brew,rwhogg/brew,rwhogg/brew,maxim-belkin/brew,Linuxbrew/brew,Linuxbrew/brew,maxim-belkin/brew,Linuxbrew/brew