Cleaned up the deprecator a little
Print warning when `paginate` is set to `true`.

Related to #1105.

Sample output:
~/code/jekyll/tsite$ ../bin/jekyll build --trace
Configuration file: /Users/parker/code/jekyll/tsite/_config.yml
    Config Warning: The `paginate` key must be a positive integer or nil. It's currently set to 'true'.
            Source: /Users/parker/code/jekyll/tsite
       Destination: /Users/parker/code/jekyll/tsite/_site
      Generating... done.
paginate option cannot be 0.
Add test for Configuration#fix_common_issues
Merge pull request #1390 from mojombo/better-errors

Catch and fix (somehow) common configuration errors