Fix Travis JRuby mode for JRuby 1.7

Since carrierwaveuploader#1754 merge, JRuby builds fails due to the new hash syntax.
See https://travis-ci.org/carrierwaveuploader/carrierwave/jobs/93603851
This is due to JRuby mode. `jruby` resolves to 1.7.10 in 1.9 mode and
`jruby-head` resolves to the JRuby master branch in 2.1 mode.
As Travis doesn't support d20 and d21 for JRuby (see travis-ci/travis-ci#2432 (comment)),
using `JRUBY_OPTS` sets the mode to run for JRuby 1.7.
This matches MRI Ruby minimal requirements for CarrierWave 1.0.0.

.travis.yml

@@ -8,6 +8,9 @@ rvm:
  - jruby
  - jruby-head
 
+env:
+  - JRUBY_OPTS="--2.0"
+
 gemfile:
   - Gemfile
   - gemfiles/rails-4-0-stable.gemfile