Fixed broken letter check

This was reintroduced in 36fb2d244e5bfbcf0af994535cd7667609766430.
Hide removed formulae revdeps
Mark removed formulae in lists

This is currently only used in formula dependencies. If you see this,
there's probably a problem in the repository, i.e. the formula can't be
installed without its dependencies.
Redirect Homebrew's Formulary.factory to the correct paths
Implement Repository#regenerate!
Improve Repository#update_metadata

Do operate on Formula objects instead of files and use formula paths
if applicable.
Do only fallback to Formula#version if no other version is set
Skip validation on relations

This fixes a rare infinite loop in Mongoid and validation is not needed
anyway.
Fixed a few code smells
Order dependencies by name
Simplified formula history layout
Only show the 10 most recent revisions of a formula
Provide more information about formula versions
Use Font Awesome icons in the revision list
Use Font Awesome icons on the front page
Add logging to about recovered formulae