Use zip-container 2.0.

Update the MetaInf classes to use the new API.
Rename UCF::Container to UCF::File.

Consistent with zip-container 2.0 and makes sense given it is a file-based
serialization.
Use released zip-container 2.0 gem.
Add a required ruby version statement to the gemspec.

Set it to ">= 1.9.3" as this is the least supported version of ruby and the
zip-container gem requires 1.9.3 as well.
Add ruby-head and rbx-2 to the Travis configuration.
Remove UCF::Container compatibility.

Update ReadMe to reflect this.
Bump version number to 1.0.0.
Update changelog for release.