Cache external link checks (ethereum#1164)

* Cache external link checks

* Cache the external checking results

* Bump the build, add sushi

* Bump build, remove sushi

* Fix SSL errors

.travis-ci.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 set -e # halt script on error
 
-HTMLPROOFER_OPTIONS="./_site --internal-domains=eips.ethereum.org --check-html --check-opengraph --report-missing-names --log-level=:debug --assume-extension --empty-alt-ignore --url-ignore=/EIPS/eip-1,EIPS/eip-1,/EIPS/eip-107,/EIPS/eip-858"
+HTMLPROOFER_OPTIONS="./_site --internal-domains=eips.ethereum.org --check-html --check-opengraph --report-missing-names --log-level=:debug --assume-extension --empty-alt-ignore --timeframe=6w --url-ignore=/EIPS/eip-1,EIPS/eip-1,/EIPS/eip-107,/EIPS/eip-858"
 
 if [[ $TASK = 'htmlproofer' ]]; then
   bundle exec jekyll doctor

.travis.yml

@@ -2,8 +2,12 @@ sudo: false # route your build to the container-based infrastructure for a faste
 
 language: ruby
 
-# Cache Ruby bundles
-cache: bundler
+cache:
+  # Cache Ruby bundles
+  - bundler
+  - directories:
+    - $TRAVIS_BUILD_DIR/tmp/.htmlproofer #https://github.com/gjtorikian/html-proofer/issues/381
+
 
 # Assume bundler is being used, therefore
 # the `install` step will run `bundle install` by default.
@@ -31,3 +35,8 @@ notifications:
     urls:
       - https://ethlab-183014.appspot.com/merge/
     on_success: always
+
+addons:
+  apt:
+    packages:
+      "libcurl4-openssl-dev" # https://github.com/gjtorikian/html-proofer/issues/376#issuecomment-332767999