Upgrade sbt-whitesource to 0.1.18

Without this Travis CI was failing:
https://travis-ci.org/github/sbt/zinc/jobs/671727552

    [error] Caused by: java.lang.RuntimeException: Multiple elements for the same key (org.scala-lang.modules,scala-xml_2.12):
    [error] 	ModuleInfo(org.scala-lang.modules,scala-xml_2.12,1.2.0,Some((Artifact(scala-xml_2.12, bundle, jar, None, Vector(), Some(https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.12/1.2.0/scala-xml_2.12-1.2.0.jar), Map(), None, false),/home/travis/.cache/coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.12/1.2.0/scala-xml_2.12-1.2.0.jar)))
    [error] 	ModuleInfo(org.scala-lang.modules,scala-xml_2.12,1.2.0,Some((Artifact(scala-xml_2.12, bundle, jar, None, Vector(), Some(https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.12/1.2.0/scala-xml_2.12-1.2.0.jar), Map(), None, false),/home/travis/.cache/coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.12/1.2.0/scala-xml_2.12-1.2.0.jar)))
    [error] 	at scala.sys.package$.error(package.scala:30)
    [error] 	at sbtwhitesource.package$KeyByAndMergeSyntax$.$anonfun$keyByAndMerge$2(package.scala:26)

This brings in the following 2 fixes:

* lightbend/sbt-whitesource#68
* lightbend/sbt-whitesource#70

project/plugins.sbt

@@ -7,7 +7,7 @@ addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.3.7")
 addSbtPlugin("de.heikoseeberger" % "sbt-header" % "3.0.2")
 addSbtPlugin("com.thesamet" % "sbt-protoc" % "0.99.31")
 libraryDependencies += "com.thesamet.scalapb" %% "compilerplugin" % "0.10.2"
-addSbtPlugin("com.lightbend" % "sbt-whitesource" % "0.1.16")
+addSbtPlugin("com.lightbend" % "sbt-whitesource" % "0.1.18")
 addSbtPlugin("com.eed3si9n" % "sbt-buildinfo" % "0.9.0")
 addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.6.1")
 addSbtPlugin("com.eed3si9n" % "sbt-projectmatrix" % "0.5.1")