[travis] allow ruby 1.9.3 failures
[travis] allow ruby 1.9.3 failures

`mime-types-data` fails to install on ruby 1.9.3

```
Installing mime-types-data 3.2016.0221
Gem::InstallError: mime-types-data requires Ruby version >= 2.0.
An error occurred while installing mime-types-data (3.2016.0221), and Bundler cannot continue.
```

see: https://travis-ci.org/justinfrench/formtastic/jobs/118493417

ryanfox1985/formtastic,justinfrench/formtastic,ryanfox1985/formtastic,ryanfox1985/formtastic,justinfrench/formtastic,justinfrench/formtastic