improves app generator bundle install

Using backticks doesn't send the output of `bundle install` to the current standard out, so using `system` instead. I like seeing what `bundle install` is doing, and guess others do too. Also, `FileUtils.cd` with a block will cd to the directory to run the commands in the block, then return to original directory.

Also, friendlier done message with hint to next step.
Merge pull request #61 from koriroys/master

improves app generator bundle install