Update devDependencies to reflect our gruntfile.

We have a lot of packages in devDependencies that are actually
used by our build:production grunt process.

Update package.json to reflect this.
Add a heroku:development task to our Gruntfile.

This is required because we're using a forked buildpack for
Grunt support on Heroku.

See https://github.com/mbuchetics/heroku-buildpack-nodejs-grunt
Update README for Heroku.
Merge pull request #528 from alphagov/enhancement/build-process-for-heroku

Build process for Heroku
Show six bars on 'bar chart with number' module.

This is so that we can show six quarters of data by default on the
high-volume bar charts.

Almost all of the high-volume services actually have data going
back six quarters (to Q4 2012) and the more we can show, the
more meaningful the graphs become.

Change font-size on the x-axis labels to 12px: this matches the
font size used for x-axis labels in journey graphs.
Bump Cheapseats version.

We've updated Cheapseats so it doesn't look explicitly for 5 bars
on the bar chart with number module: it just checks that some
bars exist.