Update license headers to 2019

[bufdev]

All tests will fail without this.

[codecov-io]

Codecov Report

Merging #262 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #262   +/-   ##
=======================================
  Coverage   70.82%   70.82%           
=======================================
  Files          90       90           
  Lines        4549     4549           
=======================================
  Hits         3222     3222           
  Misses        969      969           
  Partials      358      358

Impacted Files	Coverage Δ
internal/lint/runner.go	71.42% <ø> (ø)	⬆️
internal/protoc/protoc.go	55% <ø> (ø)	⬆️
internal/lint/check_enums_have_comments.go	100% <ø> (ø)	⬆️
internal/lint/check_enum_field_names_uppercase.go	100% <ø> (ø)	⬆️
internal/lint/check_file_options_unset.go	100% <ø> (ø)	⬆️
internal/reflect/reflect.go	100% <ø> (ø)	⬆️
.../lint/check_request_response_types_in_same_file.go	87.5% <ø> (ø)	⬆️
internal/compatible/service.go	100% <ø> (ø)	⬆️
internal/lint/check_wkt_directly_imported.go	66.66% <ø> (ø)	⬆️
internal/format/printer.go	100% <ø> (ø)	⬆️
... and 80 more

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