fix - if optipng is in $PATH

I encountered an ENOENT error while using this module with your gulp-imagemin. The reason was that i already had optipng installed an somewhere in my $PATH. In this case optipng isn't installed (correct) but bin.use() returns something like node_modules/xyz/vendors/optipng. The problematic part is that also the folder vendors doesnt exist in this case. Calling optipng in an no existent folder raises an ENOENT error thus fails.