Add .ruby-version to 1.9.3

We don't actually execute any Ruby in this project, except for during
the build process.

I think this will alleviate some issues when installing gems:

    sudo: no tty present and no askpass program specified
    Sorry, try again.

    Gem::Exception: Cannot load gem at [/var/lib/gems/1.9.1/cache/sass-3.2.12.gem]
Use bundle exec when building Sass in production

So that we use the copy of the sass binary in vendor/cache.
Use tabular numbers in more info links
Include grunt-digest in dev/prod builds

The digest task will output a JSON file into the public directory
which contains a mapping from filename to filename-cachebusted.

It will also *copy* all the assets in the public directory to their
cachebusted names.
Add cachebusting to spotlight.css

As a proof-of-concept, cachebust this one file. This involves converting
it to use underscore templates rather than mustache. Node.js templating
is painful.
Remove dependency on build:dev from test

npm test (what Travis runs) uses build:development followed by
test:all. These two tasks have some duplication, so just use test:all.

This means that test:all must be responsible for doing some additional things,
like sass:development etc.

Remove nodemon config because we don't use it since #55 was merged.
Merge pull request #113 from alphagov/cachebusting-proof-of-concept

Cachebusting proof of concept
Use tabular numbers in graph callout
Use pluralise() method in volumetrics/number view

Corrects "last 1 weeks" -> "last 1 week". Which still sounds awkward,
but is at least a little better.
Update README and CONTRIBUTING
Sort controller_map alphabetically

This does not modify any behaviour.
Sort package.json dependencies alphabetically
Update grunt and grunt-contrib dependencies
Merge pull request #114 from alphagov/use-tabular-numbers

Use tabular numbers in more info links
Merge pull request #117 from alphagov/end-of-year-tidy

End of year tidy
Change formatDuration method to use a given unit

Previously we relied on the visual length (number of characters) of
a string to determine what unit it was output as. I think we should
be able to specify the unit (milliseconds or seconds) in the method
signature.

Allows formatting to a given number of significant figures.
Merge pull request #118 from alphagov/58790312-page-load-time-seconds

Display page load time in seconds
Fix neg number bug in roundToSignificantFigures

And make it a method on View.
Move rounding changes tests into correct place

And remove one duplicate.

And tidy whitespace on an object, remove unused thresholds (they're
duplicated below).
Change rounding thresholds from 500 to 1000

For all orders of magnitude.

I think there's an opportunity for some refactoring here.