e2e-tests: Introduce reused gatling version constant

Change-Id: I5e95b7938c4527e506a9267d8f48c776d1ab3502
diff --git a/e2e-tests/build.sbt b/e2e-tests/build.sbt
index 685db37..9c49ed2 100644
--- a/e2e-tests/build.sbt
+++ b/e2e-tests/build.sbt
@@ -13,6 +13,6 @@
       name := "gerrit",
       libraryDependencies ++=
           gatling ++
-              Seq("io.gatling" % "gatling-core" % "3.1.1") ++
-              Seq("io.gatling" % "gatling-app" % "3.1.1")
+              Seq("io.gatling" % "gatling-core" % GatlingVersion) ++
+              Seq("io.gatling" % "gatling-app" % GatlingVersion)
     ) dependsOn gatlingGitExtension
diff --git a/e2e-tests/project/Dependencies.scala b/e2e-tests/project/Dependencies.scala
index 72d2ac2..62d1f74 100644
--- a/e2e-tests/project/Dependencies.scala
+++ b/e2e-tests/project/Dependencies.scala
@@ -1,8 +1,10 @@
 import sbt._
 
 object Dependencies {
+  val GatlingVersion = "3.1.1"
+
   lazy val gatling = Seq(
     "io.gatling.highcharts" % "gatling-charts-highcharts",
     "io.gatling" % "gatling-test-framework",
-  ).map(_ % "3.1.1" % Test)
+  ).map(_ % GatlingVersion % Test)
 }