put only a single copy of gadfly.js and snap.svg-min.js in text/html output

[bjarthur]

title says it all. a copy is already put in by the preceding call to SVGJS.

[codecov-io]

Codecov Report

Merging #1259 into master will increase coverage by 4.99%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1259      +/-   ##
==========================================
+ Coverage   85.34%   90.33%   +4.99%     
==========================================
  Files          36       36              
  Lines        4148     3943     -205     
==========================================
+ Hits         3540     3562      +22     
+ Misses        608      381     -227

Impacted Files	Coverage Δ
src/Gadfly.jl	78.98% <ø> (+4.46%)	⬆️
src/geom/bar.jl	93.23% <0%> (+1.38%)	⬆️
src/shapes.jl	100% <0%> (+1.96%)	⬆️
src/geom/polygon.jl	94.87% <0%> (+2.37%)	⬆️
src/mapping.jl	85.43% <0%> (+2.41%)	⬆️
src/geom/segment.jl	94.44% <0%> (+2.55%)	⬆️
src/guide.jl	87.87% <0%> (+3.48%)	⬆️
src/guide/keys.jl	94.73% <0%> (+3.59%)	⬆️
src/coord.jl	88.17% <0%> (+3.63%)	⬆️
src/color_misc.jl	62.06% <0%> (+4%)	⬆️
... and 14 more

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