Pin the nokogiri version (#362)

Workaround for 1.8.0 not building on Jenkins and masking the errors

Gemfile

@@ -2,5 +2,6 @@ source "https://rubygems.org"
 
 gem "github-pages", group: :jekyll_plugins
 gem "jekyll-include-cache", "~> 0.1"
+gem "nokogiri", "= 1.7.2"
 gem "html-proofer"
 

Gemfile.lock

@@ -22,7 +22,7 @@ GEM
     ffi (1.9.18)
     forwardable-extended (2.6.0)
     gemoji (3.0.0)
-    github-pages (138)
+    github-pages (139)
       activesupport (= 4.2.8)
       github-pages-health-check (= 1.3.3)
       jekyll (= 3.4.3)
@@ -51,7 +51,7 @@ GEM
       jekyll-theme-midnight (= 0.0.4)
       jekyll-theme-minimal (= 0.0.4)
       jekyll-theme-modernist (= 0.0.4)
-      jekyll-theme-primer (= 0.1.8)
+      jekyll-theme-primer (= 0.2.1)
       jekyll-theme-slate (= 0.0.4)
       jekyll-theme-tactile (= 0.0.4)
       jekyll-theme-time-machine (= 0.0.4)
@@ -73,16 +73,16 @@ GEM
     html-pipeline (2.6.0)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.6.0)
+    html-proofer (3.7.2)
       activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
       colored (~> 1.2)
       mercenary (~> 0.3.2)
-      nokogiri (~> 1.5)
+      nokogiri (~> 1.7)
       parallel (~> 1.3)
       typhoeus (~> 0.7)
       yell (~> 2.0)
-    i18n (0.8.1)
+    i18n (0.8.4)
     jekyll (3.4.3)
       addressable (~> 2.4)
       colorator (~> 1.0)
@@ -147,7 +147,7 @@ GEM
       jekyll (~> 3.3)
     jekyll-theme-modernist (0.0.4)
       jekyll (~> 3.3)
-    jekyll-theme-primer (0.1.8)
+    jekyll-theme-primer (0.2.1)
       jekyll (~> 3.3)
     jekyll-theme-slate (0.0.4)
       jekyll (~> 3.3)
@@ -180,7 +180,7 @@ GEM
       mini_portile2 (~> 2.1.0)
     octokit (4.7.0)
       sawyer (~> 0.8.0, >= 0.5.3)
-    parallel (1.11.1)
+    parallel (1.11.2)
     pathutil (0.14.0)
       forwardable-extended (~> 2.6)
     public_suffix (2.0.5)
@@ -210,6 +210,7 @@ DEPENDENCIES
   github-pages
   html-proofer
   jekyll-include-cache (~> 0.1)
+  nokogiri (= 1.7.2)
 
 BUNDLED WITH
    1.14.6