DataFrames_0.2 compatibility

[Mattriks]

• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• adds DataFrames 0.2.0 compatibility
• fixes Function flatten conflict with exported function flatten from DataFrames #1359

[codecov-io]

Codecov Report

Merging #1360 into master will not change coverage.
The diff coverage is 87.5%.

@@          Coverage Diff           @@
##           master   #1360   +/-   ##
======================================
  Coverage    90.6%   90.6%           
======================================
  Files          38      38           
  Lines        3961    3961           
======================================
  Hits         3589    3589           
  Misses        372     372

Impacted Files	Coverage Δ
src/data.jl	0% <0%> (ø)	⬆️
src/geom/ribbon.jl	100% <100%> (ø)	⬆️
src/geom/boxplot.jl	100% <100%> (ø)	⬆️
src/scale.jl	97.41% <100%> (ø)	⬆️
src/Gadfly.jl	78.14% <100%> (ø)	⬆️
src/statistics.jl	96.88% <100%> (ø)	⬆️

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