Only require used active_support extensions.

Libraries should never modify core classes indirectly. Explicitly
require only used active_support extensions rather than loading (or
autoloading) everything.
Merge pull request #78 from shanna/2-7-stable-explicit_active_support

Only require used active_support extensions.
Bump version 2.7.7
Create CONTRIBUTING.md
Fix travis zmq install, add ruby 2.0 build
Fix StringField#encode to specify from/to encodings for ruby 2.0
Merge branch 'ruby_2.0'