Avoid dalli 2.7.7 for now

It's causing a test to fail.
Ref: https://travis-ci.org/rails/rails/jobs/353758855#L1433-L1501

Gemfile

@@ -45,7 +45,7 @@ group :doc do
 end
 
 # Active Support.
-gem "dalli", ">= 2.2.1"
+gem "dalli", "2.7.6"
 gem "listen", ">= 3.0.5", "< 3.2", require: false
 gem "libxml-ruby", platforms: :ruby
 gem "connection_pool", require: false

Gemfile.lock

@@ -510,7 +510,7 @@ DEPENDENCIES
   chromedriver-helper
   coffee-rails
   connection_pool
-  dalli (>= 2.2.1)
+  dalli (= 2.7.6)
   delayed_job
   delayed_job_active_record
   google-cloud-storage (~> 1.8)