upgrade to latest rspec version

this includes removing deprecated syntax.
this is to avoid upgrade debt.
bump uglifier

this will make the rails 4 migration easier
bump exception_notification gem version to 4.0.0

this will make the rails4 upgrade easier
bump webmock gem version

this is to avoid upgrade debt.
bump plek gem

this is to avoid upgrade debt in the future
Merge pull request #60 from alphagov/bump-gems-to-reduce-upgrade-debt

Bump gems to reduce upgrade debt, simplify future rails4 migration