Use Pathname#to_s instead of Pathname#to_path

Fixes #1723.
Merge pull request #1730 from mojombo/pathname-to-s

Use Pathname#to_s instead of Pathname#to_path
Update history to reflect merge of #1730
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
Update link to example Rakefile to point to specific commit.

The link was pointing to the latest version of the file which had been changed and no longer provided the intended example.
Merge pull request #1741 from andrewhavens/patch-1

Update link to example Rakefile to point to specific commit.
Update history to reflect merge of #1741