Rakefile: print a warning if mechanize is not installed
Rakefile: print a warning if mechanize is not installed

Check as part of "prerelease" task. Will save me from having the upload
task explode in my face and having to re-run it every time I switch to a
new machine, or a new Ruby version.

yrro/command-t,wincent/command-t,mtimkovich/command-t,yrro/command-t,mtimkovich/command-t,wincent/command-t,wincent/command-t