Bump govuk_frontend_toolkit to 5.1.1

# 5.1.1

- Update the alpha, beta and discovery colours to $govuk-blue to match
the updated phase banner ([PR
alphagov#370](alphagov/govuk_frontend_toolkit#370))
- Fix radio button show/hide behaviour when used outside a form ([PR
alphagov#375](alphagov/govuk_frontend_toolkit#375))
- Fix a "Stick at top when scrolling" component bug related to scroll
anchoring in Chrome 56+ ([PR
alphagov#376](alphagov/govuk_frontend_toolkit#376))
- Minor travis fixes ([PR
alphagov#373](alphagov/govuk_frontend_toolkit#373))

# 5.1.0

- Allow custom options when tracking events ([PR
alphagov#365](alphagov/govuk_frontend_toolkit#365))

# 5.0.3

- Change HMRC and DEFRA text colours for improved contrast ([PR
alphagov#368](alphagov/govuk_frontend_toolkit#368))

package.json

@@ -21,7 +21,7 @@
     "express-session": "^1.13.0",
     "express-writer": "0.0.4",
     "govuk-elements-sass": "2.2.1",
-    "govuk_frontend_toolkit": "5.0.2",
+    "govuk_frontend_toolkit": "5.1.1",
     "govuk_template_jinja": "0.19.2",
     "gulp": "^3.9.1",
     "gulp-clean": "^0.3.2",