Add breaking scenario and spec for #520
Fixed #520: fix incorrect html formatter behaviour when background step fails

When encountering a failure in a background step, the html formatter
incorrectly marked the previous scenario red. With this fix, the html
formatter now correctly marks the current background red and does not
touch the output of the previous scenario anymore.
Fixed #522: Do not print superfluous and badly formatted backtrace when failing step is a background step
warning messages added
Use the less brittle gherkin language help feature from master
Bump gherkin version.
warning message of AST::Table#map_headers! changed
map_headers refactored
map_headers! specs separated of map_headers specs
added block to map_headers
map_headers changed
Merge pull request #523 from mlex/html-formatter-bad-backtrace-for-failing-background

Fixed #522: Do not print superfluous and badly formatted backtrace when ...
Merge pull request #525 from adbatista/deprecation_warnings_AST_TABLE

warning messages added
Merge pull request #528 from adbatista/refactor_map_headers

map_headers refactored
Merge pull request #521 from mlex/html-formatter-incorrect-for-failing-background-step

Html formatter incorrect for failing background step
Release 1.3.7
Merge branch 'v1.3.x-bugfix'

Conflicts:
	History.md
	features/docs/defining_steps/using_table_mapping.feature
	features/lib/step_definitions/language_steps.rb
	lib/cucumber/platform.rb