Update CONTRIBUTING.md

Add some helpful hints for those new to pg.
Bump to 2.1.5
Update newrelic_rpm dependency to version 3.6.7.159
remove references to humanmsg.css and humanmsg.js
since these files don't exist anymore
Update webmock dependency to version 1.14.0
Update multi_json dependency to version 1.8.1
Update redis dependency to version 3.0.5
Merge pull request #604 from dwradcliffe/fix-404

remove references to humanmsg.css and humanmsg.js
Merge pull request #600 from knappe/patch-2

Update CONTRIBUTING.md
Update high_voltage dependency to version 2.0.0
Bump to 2.1.6
Merge branch 'master' of github.com:rubygems/rubygems.org
Bump to 2.1.7
Update database_cleaner to version 1.2.0
Update newrelic_rpm dependency to version 3.6.8.164
Update gherkin dependency to version 2.12.2