Use authentification for github release API requests

So far the requests against the Github API to fetch release information
has been made unauthenticated, making them subject to a rate-limit of 10
requests per hour per IP. As the build process is executed on Travis CI,
this results in many build failures.

The requests are now made with a personal access token, in the form of
GITHUB_AUTHENTICATION="-u prombot:<oauth token>". The standard printing
in the Makefile has been removed in order to not expose the token.

.travis.yml

 sudo: false
 language: ruby
-
 branches:
-    only:
-        - master
+  only:
+  - master
 script: make deploy
 before_install:
-    - eval "$(ssh-agent -s)"
-    - openssl aes-256-cbc -K $encrypted_2ba894bc7c2f_key -iv $encrypted_2ba894bc7c2f_iv -in prometheus_rsa.enc -out prometheus_rsa -d
-    - chmod 600 prometheus_rsa
-    - ssh-add prometheus_rsa
+- eval "$(ssh-agent -s)"
+- openssl aes-256-cbc -K $encrypted_2ba894bc7c2f_key -iv $encrypted_2ba894bc7c2f_iv -in prometheus_rsa.enc -out prometheus_rsa -d
+- chmod 600 prometheus_rsa
+- ssh-add prometheus_rsa
+env:
+  global:
+    secure: aNZoB4UDtdelUDxFTTJa5Dxm8gXorKGmQWYaFIXzuBwLvFMp1toIY+amsMrGBbnvxrJosO4764GpS8sWRT/Jbr252sxjOe8N4jfbtJUX29U2aNFQSWVc/o8VnKFW1NI5cjBAZ9OOvgpS6KUGkXZ2/PTmvgxvyqGS4pPovIM1OOo=

Makefile

@@ -11,12 +11,14 @@ deploy: github_pages_export github_pages_push
 downloads: $(DOWNLOADS:%=downloads/%/repo.json) $(DOWNLOADS:%=downloads/%/releases.json)
 
 downloads/%/repo.json:
-	mkdir -p $(dir $@)
-	curl -sf -H 'Accept: application/vnd.github.v3+json' https://api.github.com/repos/prometheus/$* > $@
+	@mkdir -p $(dir $@)
+	# curl -sf -H 'Accept: application/vnd.github.v3+json' <GITHUB_AUTHENTICATION> https://api.github.com/repos/prometheus/$* > $@
+	@curl -sf -H 'Accept: application/vnd.github.v3+json' $(GITHUB_AUTHENTICATION) https://api.github.com/repos/prometheus/$* > $@
 
 downloads/%/releases.json:
-	mkdir -p $(dir $@)
-	curl -sf -H 'Accept: application/vnd.github.v3+json' https://api.github.com/repos/prometheus/$*/releases > $@
+	@mkdir -p $(dir $@)
+	# curl -sf -H 'Accept: application/vnd.github.v3+json' <GITHUB_AUTHENTICATION> https://api.github.com/repos/prometheus/$*/releases > $@
+	@curl -sf -H 'Accept: application/vnd.github.v3+json' $(GITHUB_AUTHENTICATION) https://api.github.com/repos/prometheus/$*/releases > $@
 
 github_pages_export: compile
 	cd output && \