Fixes bug in formula for wkhtmltopdf.

Formula for wkhtmltopdf was passing inreplace a Symbol where it
expected a String.

Error: can't convert Symbol into String
/usr/local/Library/Homebrew/utils/inreplace.rb:12:in `gsub!'
/usr/local/Library/Homebrew/utils/inreplace.rb:12:in `inreplace'
/usr/local/Library/Homebrew/utils/inreplace.rb:4:in `each'
/usr/local/Library/Homebrew/utils/inreplace.rb:4:in `inreplace'
/usr/local/Library/Formula/wkhtmltopdf.rb:23:in `install'
/usr/local/Library/Homebrew/build.rb:170:in `install'
/usr/local/Library/Homebrew/formula.rb:239:in `brew'
/usr/local/Library/Homebrew/formula.rb:600:in `stage'
/usr/local/Library/Homebrew/extend/fileutils.rb:21:in `mktemp'
/usr/local/Library/Homebrew/formula.rb:596:in `stage'
/usr/local/Library/Homebrew/formula.rb:234:in `brew'
/usr/local/Library/Homebrew/build.rb:149:in `install'
/usr/local/Library/Homebrew/build.rb:43:in `main'
/usr/local/Library/Homebrew/build.rb:12
/usr/local/Library/Formula/wkhtmltopdf.rb:11