RubyGems update and JRuby 1.7.5

In response to intermittent SSL-related errors encountered during the `bundle install` process on Semaphore's build platform, the RubyGems package was upgraded to version 2.1.9 across all provided Rubies. The issue, which affected users sourcing rubygems.org via HTTPS, was characterized by SSL certificate verification failures, potentially indicating a missing CA certificate. While the problem was challenging to debug due to its random occurrence, the upgrade appears to have resolved it, with further monitoring and potential backporting of the ca-certificates package being considered if required. A temporary workaround involves switching the Gemfile source to HTTP. Additionally, JRuby was updated to version 1.7.5, and efforts were made to align SSL configurations, including setting the SSL_CERT_FILE environment variable and creating necessary symlinks. The issue was ultimately rectified with a fix on the RubyGems infrastructure.