use File#write instead of File::write that isn't supported by ruby-1.9.2

File::write is not supported by ruby-1.9.2. 
Changing code like 
File.write(filename, file_content) 
to 
File.open(filename, 'w') { |file| file.write(file_content) } 
will solve the problem and still work for higher ruby versions.

We didn't experience any other issues while using teabag with ruby-1.9.2 (we are using capybara 2.0.3). But maybe if you don't plan to support ruby-1.9.2, it should be specified in the docs.