Enable publishing core + macros for 2.12

[ckchow]

• bumps sbt version to 1.1.0.
• adds explicit dep on scala-xml, which was preventing a build on 2.12 (and also a weird bug in the scala compiler ).
• bumps various library versions. enables cross-compile of base vegas module on 2.12.

[codecov-io]

Codecov Report

Merging #127 into master will decrease coverage by 68.99%. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #127     +/-   ##
=======================================
- Coverage   68.99%     0%    -69%     
=======================================
  Files          28     27      -1     
  Lines        1819   1780     -39     
  Branches        3      6      +3     
=======================================
- Hits         1255      0   -1255     
- Misses        564   1780   +1216

Impacted Files	Coverage Δ
core/src/main/scala/vegas/DSL/package.scala	0% <0%> (-100%)	:arrow_down:
spark/src/main/scala/vegas/sparkExt/package.scala	0% <0%> (-100%)	:arrow_down:
core/src/main/scala/vegas/spec/package.scala	0% <0%> (-100%)	:arrow_down:
core/src/main/scala/vegas/DSL/ScaleDSL.scala	0% <0%> (-100%)	:arrow_down:
...c/main/scala/vegas/render/StaticHTMLRenderer.scala	0% <0%> (-100%)	:arrow_down:
...e/src/main/scala/vegas/data/ValueTransformer.scala	0% <0%> (-100%)	:arrow_down:
core/src/main/scala/vegas/DSL/SortDSL.scala	0% <0%> (-100%)	:arrow_down:
core/src/main/scala/vegas/data/External.scala	0% <0%> (-100%)	:arrow_down:
...ore/src/main/scala/vegas/data/FieldExtractor.scala	0% <0%> (-100%)	:arrow_down:
core/src/main/scala/vegas/DSL/DataDSL.scala	0% <0%> (-100%)	:arrow_down:
... and 16 more

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[dbtsai]

LGTM.