Resolve bundler warning about duplicate dep

"govuk_admin_template at /Users/<user>/govuk/govuk_admin_template did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on rails (= 3.2.18, development), (>= 3.2.0) use:
    add_runtime_dependency 'rails', '= 3.2.18', '>= 3.2.0'"
Don't specify the patchlevel in .ruby-version

* Let rbenv-alias take the strain
Allow patchlevels for runtime deps

* bootstrap-sass was pinned at an older version with  problems related to fingerprinted asset urls
* be consistent with jquery-rails
Minimum Ruby version of 1.9.3
Use image-url(...) over url(...)

* We can't reference non-digest asset names in Rails 4
Load bootstrap-sprockets before bootstrap

* Fixes the incorrect generation of non-MD5 asset names
* Allows the bootstrap glyphicons to load correctly in Rails 4 apps
Move :publish_gem rake task into lib/tasks

* precursor to adding sass:check, which might otherwise inflate the Rakefile
  unduly if lib/tasks didn't exist
Add a sass:check task as part of the build

* The build will break if there are unguarded url() usages that would result
  in asset 404s for production Rails 4 apps.
Merge pull request #42 from alphagov/fingerprint-urls

Rails 4-compatible digest asset URL references