Release prep, support sbt 1.2.8 again

Author: dwijnand

.travis.yml

@@ -4,7 +4,8 @@ scala: 2.12.10
 
 jobs:
   include:
-  - { script: sbt mimaReportBinaryIssues test IntegrationTest/test }
+  - { script: "sbt mimaReportBinaryIssues test IntegrationTest/test
+        'set sbtplugin/scriptedSbt := \"1.2.8\"' 'scripted sbt-mima-plugin/minimal'" }
   - { name: testFunctional 2.11, script: sbt -Dmima.testScalaVersion=2.11.12 testFunctional }
   - { name: testFunctional 2.12, script: sbt -Dmima.testScalaVersion=2.12.10 testFunctional }
   - { name: testFunctional 2.13, script: sbt -Dmima.testScalaVersion=2.13.1  testFunctional }

README.md

@@ -36,13 +36,12 @@ source compatibility.
 
 ## Usage
 
-The latest version of MiMa's sbt plugin supports sbt 1.3.0+.
-Use v0.6.0 for sbt 1.0.x - 1.2.x, and v0.3.0 for sbt 0.13.x.
+MiMa's sbt plugin supports sbt 1.x only.  Use v0.3.0 for sbt 0.13.x.
 
 To use it add the following to your `project/plugins.sbt` file:
 
 ```
-addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.6.1")
+addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.6.2")
 ```
 
 Add the following to your `build.sbt` file:

sbtplugin/src/main/scala/com/typesafe/tools/mima/plugin/MimaKeys.scala

@@ -2,7 +2,7 @@ package com.typesafe.tools.mima
 package plugin
 
 import com.typesafe.tools.mima.core.{ Problem, ProblemFilter }
-import sbt._
+import sbt.{ File, ModuleID, settingKey, taskKey } // no sbt._, to avoid 1.3+ only singleFileJsonFormatter
 
 object MimaKeys extends MimaKeys