Remove all the docs for migrations and link to http://import.jekyllrb.com
Merge pull request #1732 from mojombo/fix-import-docs

Remove all the docs for migrations and link to http://import.jekyllrb.com
Update history to reflect merge of #1732
Enable Travis' bundle caching

It should speed up the builds significantly. See
http://about.travis-ci.org/docs/user/caching/#Bundler for more details.
Merge pull request #1734 from gregkare/travis_cache

Enable Travis' bundle caching
Update history to reflect merge of #1734