Several changes:
- TravisCI
- httpd2x to always use apr, apr-util, and zlib from homebrew. Prevents errors
  from when Xcode is installed but com.apple.pkg.CLTools_Executables
  (Command Line Tools package) is not installed. Also prevents logic problems
  if installing modules like mod_security where you would have needed the same
  "--with-brewed-*" options as httpd2x
- Simplify dependencies
- Refuse to build modules if both "--with-brewed-httpd22" and "--with-brewed-httpd24"
  are present
- If not using "--with-brewed-httpd2x" with modules, on 10.9, and using Xcode
  without com.apple.pkg.CLTools_Executables, show an error and explain why it
  will fail, rather than allowing a failed build with no instructions to fix
- Formatting changes