Optimize Post#{next,previous}

Use object equality for comparisons rather than Comparable#==,
which in turn uses Post#<=>, which is slow.

This yielded a 3x performance improvement for `jekyll build`
on a large site (1m6.467s -> 0m19.532s).
Merge pull request #1983 from jfirebaugh/perf
Update history to reflect merge of #1983
Add some rebund :heart: to speed up TravisCI builds.
Don't need to install bundler, right?
Merge pull request #1985 from jekyll/rebund
Update history to reflect merge of #1985
Hm... I wonder if yarp can speed up our build process.
Merge pull request #1984 from jekyll/yarp
Update history to reflect merge of #1984