Handled single-value .sum's gracefully.

Single-value-sums now act like an identity function
instead of outputting invalid CDEFs. Further details
are in the test file
Added missing import
Mocked RRD dependency.

I didn't notice because I have that package installed
And another fix for travis.

Sorry for making this such a mess, first time with these test classes.
Merge pull request #132 from Tetha/single_value_sum

Handled single-value .sum's gracefully.