Add macro to the list of ScalaKeywords

[avibryant]

The identifier macro is now a keyword in Scala. This adds it to the list of ScalaKeywords so that it gets escaped properly by Scrooge.

[kevinoliver]

lgtm, thanks @avibryant

[codecov-io]

Codecov Report

Merging #276 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #276   +/-   ##
========================================
  Coverage    45.41%   45.41%           
========================================
  Files           73       73           
  Lines         3312     3312           
  Branches       215      221    +6     
========================================
  Hits          1504     1504           
  Misses        1808     1808

Impacted Files	Coverage Δ
...a/com/twitter/scrooge/backend/ScalaGenerator.scala	46.98% <ø> (ø)	⬆️

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[kevinoliver]

this got merged in 3251125

thanks again, @avibryant