Tinkered with model compilation

The Sankey data and the Primary Energy data now appear in the model output.

The critical things were:

1. In the Rakefile rename 'Flows' to 'flows' to match the lowercase worksheet name in the excel
2. In the spreadsheet, rename the 'Intermediate output ' worksheet to remove the trailing space
3. Update excel_to_code to 0.2.3 in the Gemfile

Updating the excel_to_code may not have been essential, but it compiles a lot quicker than the previous version.

Updating to Ruby 2.1 also makes compilation much quicker (at least on my machine).

Tom