Pins Ruby version to 2.5.3 in Travis
Pins Ruby version to 2.5.3 in Travis

Ruby 2.5.4 was recently released, and appears to have problems with bundler. We
previously pinned Ruby at 2.5 in Travis, and Travis is now resolving this to
the "broken" 2.5.4. We pin to 2.5.3 to maintain passing builds.

Not a long term fix, just a temporary fix to keep builds green.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf