Ignore distressingly-frequent segfaults on this build

Not sure what's causing them suddenly, but it seems unlikely there's
much we can do about it. For the small amount of coverage this
particular job gains us, it's hurting us more, by making CI failures
unremarkable.

.travis.yml

@@ -18,6 +18,8 @@ env:
   - "GEM=ar:postgresql"
 matrix:
   allow_failures:
+    - rvm: 1.9.3
+      env: "GEM=ar:mysql"
     - rvm: rbx-2
     - rvm: jruby
     - rvm: ruby-head