Implement fetch_multi for parity with current Rails

As of Rails 4.1.X the base ActiveSupport::Cache implementation has a
`fetch_mutli` method. This ports that method over into the dalli_store.
Reading and writing are pipelined within two separate blocks for
efficiency.
Merge pull request #380 from sorentwo/fetch-multi

Implement fetch_multi for parity with current Rails
Add support for `version` command.
Merge pull request #384 from memcachier/master

Add support for `version` command.
Fixes broken API documentation link

The rubydoc.info docs were last generated on Oct 3rd, so fairly recent.
Merge pull request #391 from tpitale/patch-1

Fixes broken API documentation link
Call to failure! requires an exception now
Merge pull request #392 from davidguthu/fix-failure-method-call

Call to failure! requires an exception now
Lower minitest dependency to >= 4.2.0.
Merge pull request #395 from benmanns/dependency-fixes

Lower minitest dependency to >= 4.2.0.
Add Code Climate badge to README
Merge pull request #396 from mrb/patch-1

Add Code Climate badge to README
add a note about new compressors. closes #406
Merge pull request #407 from pitr/patch-1

add a note about new compressors. closes #406
less encryptic error message when version is < 1.4

@error and @msg are both nil, thus the message says it's down
even though you've verified it is definitely up...something is lying!
Then you read the docs.
Merge pull request #414 from shaiguitar/master

less encryptic error message when version is < 1.4
Add better support for safe updates with multiple writers
Merge pull request #398 from philipmw/get-set-with-cas

Add get-with-cas ('gets') / set-with-CAS functionality
Cleanup
Trim helper junk from the public API