Switch from Travis to Github Actions CI (#2041)

* Fix .run_formal_checks for Github Actions instead of Travis
* Remove .run_chisel_tests

Because we publish SNAPSHOTs on every merge to master, the Chisel repo
checks against master of FIRRTL regularly

* Only run unidoc for Scala 2.12

Under Travis we ran for 2.11, 2.12, and 2.13, but it doesn't work when
using ++2.<major>.<minor> when major != to 2.12. It seems if we want to
run all 3, we have to run as `sbt +unidoc`. It's not clear how much
benefit we get from building on the other versions, so stick with 2.12
for now.

* Generate .mergify.yml with script in chisel-repo-tools

Generated with config:

conditions:
  - status-success=all tests passed
branches:
  - 1.2.x
  - 1.3.x
  - 1.4.x

1 files changed, 5 insertions, 0 deletions

diff --git a/build.sbt b/build.sbt
index 14aafe1b..f5a6a611 100644
--- a/build.sbt
+++ b/build.sbt
@@ -55,6 +55,10 @@ lazy val commonSettings = Seq(
   )
 )
 
+lazy val mimaSettings = Seq(
+  mimaPreviousArtifacts := Set()
+)
+
 lazy val protobufSettings = Seq(
   sourceDirectory in ProtobufConfig := baseDirectory.value / "src" / "main" / "proto",
   protobufRunProtoc in ProtobufConfig := (args =>
@@ -179,6 +183,7 @@ lazy val firrtl = (project in file("."))
     buildInfoUsePackageAsPath := true,
     buildInfoKeys := Seq[BuildInfoKey](buildInfoPackage, version, scalaVersion, sbtVersion)
   )
+  .settings(mimaSettings)
 
 lazy val benchmark = (project in file("benchmark"))
   .dependsOn(firrtl)