Test correct formatting of fractional placement

Only those placements which have fractions should display decimal
points. It isn't pretty to have a whole bunch of "1.0", "2.0", "3.0"
entries when there are no decimals to be seen.
Upgrade to Ruby 2.1.0

Needed for better refinement support. This is kind of a "nice to have"
not "need to have".
format_rule_cell: Take an int as an argument

Now it should be invoked as `format_rule_cell(placement.rule)`
Add `DecimalTruncateToString` with refinements

Allows easy printing of decimals as strings with their fractional part
truncated.
Re-align cucumber table
Test display of fractional placements

This checks whether couples with fractional placements are correctly
displayed in HTML.
Add `to_st` refinement to Fixnum

This way, both `Placement` and `SubPlacement` models can have `to_st`
called on them easily.
Do not create duplicate final rounds in factory

If a final round is created using Factory Girl, it will not increment
and create a new round number. This way, repeated invocations of the
following invocation will only create a single `Round` (for each event):

    FactoryGirl.create(:round, event: @event, final: true)
Use `BigDecimal.to_st` in helpers and views
Put `data_table_helper` into a module

This lets it use refinements without polluting the global namespace.
Fix fractional placement features
Misc Rubocop-inspired fixes

Small things like re-indentation or line-length limits.
Merge branch 'fractional-placement-fix'