Fix #save base spec

It was failing on Ruby 2.0 because of initialization problems
when using #dup.
Disable caching only in Rails' test mode (by default)
Set default expiration time when caching to 10 minutes
Set cache expiration to 60 seconds in Rails' development

i.e. when Amfetamine is included in a Rails application and the
application is run in development mode.
Add caching information to README
Test on Ruby 2.0.0 [travis]
Drop Ruby 1.9.2 support from travis

1.9.2 is dead.
Fix #to_json spec

It was failing with latest gem versions (most likely it's about the json
gem).
Remove .rspec
Fix base spec
Merge pull request #11 from Exvo/caching_settings

Set default cache settings in Dalli::Client