Update gds-api-adapters from v25.1.0 to v30.6.0

I noticed that we were using quite an old version of gds-api-adapters and I
thought it would be sensible to get more up-to-date.

Also I'm contemplating pushing some functionality out of the smart-answers app
and into gds-api-adapters. Being up-to-date with gds-api-adapters will make this
easier.

I looked through CHANGELOG for gds-api-adapters and didn't see anything that
worried me. Most of the changes seemed to be additions to the API.

I made the change to version of gds-api-adapters in the Gemfile and ran
`bundle update gds-api-adapters`. I then inspected the changes to Gemfile.lock
and removed the following unnecessary changes:

* domain_name (v0.5.25 -> v0.5.20160310)
* mime-types (v2.99.1 -> v2.99.2)
* rack-cache (v1.5.1 -> v1.6.1)
* unf_ext (v0.0.7.1 -> v0.0.7.2)

I've run all the tests including the regression tests and they all pass.

Note that I've stuck with constraining gds-api-adapters using the
`~> major-version.minor-version` style. We seem to be inconsistent about
how we approach the constraints on gems in the Gemfile, but I'm not going to
worry about that in this PR.

Gemfile

@@ -6,7 +6,7 @@ gem 'rails', '4.2.6'
 
 gem 'airbrake', '4.1.0'
 gem 'ast'
-gem 'gds-api-adapters', '~> 25.1'
+gem 'gds-api-adapters', '~> 30.6'
 gem 'govspeak', '~> 3.3.0'
 gem 'govuk-content-schema-test-helpers', '~> 1.3.0'
 gem 'govuk_frontend_toolkit', '4.8.0'

Gemfile.lock

@@ -71,11 +71,11 @@ GEM
       unf (>= 0.0.5, < 1.0.0)
     erubis (2.7.0)
     execjs (2.4.0)
-    gds-api-adapters (25.1.0)
+    gds-api-adapters (30.6.0)
       link_header
       lrucache (~> 0.1.1)
       null_logger
-      plek
+      plek (>= 1.9.0)
       rack-cache
       rest-client (~> 1.8.0)
     globalid (0.3.6)
@@ -264,7 +264,7 @@ DEPENDENCIES
   binding_of_caller
   capybara (= 2.1.0)
   ci_reporter
-  gds-api-adapters (~> 25.1)
+  gds-api-adapters (~> 30.6)
   govspeak (~> 3.3.0)
   govuk-content-schema-test-helpers (~> 1.3.0)
   govuk-lint