(maint) pin i18n

activesupport calls in the latest i18n, which drops support for Ruby
1.8.7 and breaks acceptance tests on Ubuntu 12.04.  This PR pins to a lower
version with 1.8.7 support.
Merge pull request #1188 from wkalt/maint/pin-i18n

(maint) activesupport calls in the latest i18n, which drops support for ...