Update resolvers in build.sbt to contain Maven as a dependency

[adl1995]

When trying to build Cortex from source on CentOS 7, I was getting this error:

[error] sbt.librarymanagement.ResolveException: unresolved dependency: org.elasticsearch.client#x-pack-transport;5.6.2: not found
[error] 	at sbt.internal.librarymanagement.IvyActions$.resolveAndRetrieve(IvyActions.scala:334)
[error] 	at sbt.internal.librarymanagement.IvyActions$.$anonfun$updateEither$1(IvyActions.scala:208)
[error] 	at sbt.internal.librarymanagement.IvySbt$Module.$anonfun$withModule$1(Ivy.scala:243)
[error] 	at sbt.internal.librarymanagement.IvySbt.$anonfun$withIvy$1(Ivy.scala:204)
[error] 	at sbt.internal.librarymanagement.IvySbt.sbt$internal$librarymanagement$IvySbt$$action$1(Ivy.scala:70)
[error] 	at sbt.internal.librarymanagement.IvySbt$$anon$3.call(Ivy.scala:77)
[error] 	at xsbt.boot.Locks$GlobalLock.withChannel$1(Locks.scala:95)
[error] 	at xsbt.boot.Locks$GlobalLock.xsbt$boot$Locks$GlobalLock$$withChannelRetries$1(Locks.scala:80)
[error] 	at xsbt.boot.Locks$GlobalLock$$anonfun$withFileLock$1.apply(Locks.scala:99)
[error] 	at xsbt.boot.Using$.withResource(Using.scala:10)
[error] 	at xsbt.boot.Using$.apply(Using.scala:9)
[error] 	at xsbt.boot.Locks$GlobalLock.ignoringDeadlockAvoided(Locks.scala:60)
[error] 	at xsbt.boot.Locks$GlobalLock.withLock(Locks.scala:50)
[error] 	at xsbt.boot.Locks$.apply0(Locks.scala:31)
[error] 	at xsbt.boot.Locks$.apply(Locks.scala:28)
[error] 	at sbt.internal.librarymanagement.IvySbt.withDefaultLogger(Ivy.scala:77)
[error] 	at sbt.internal.librarymanagement.IvySbt.withIvy(Ivy.scala:199)
[error] 	at sbt.internal.librarymanagement.IvySbt.withIvy(Ivy.scala:196)
[error] 	at sbt.internal.librarymanagement.IvySbt$Module.withModule(Ivy.scala:242)
[error] 	at sbt.internal.librarymanagement.IvyActions$.updateEither(IvyActions.scala:193)
[error] 	at sbt.librarymanagement.ivy.IvyDependencyResolution.update(IvyDependencyResolution.scala:20)
[error] 	at sbt.librarymanagement.DependencyResolution.update(DependencyResolution.scala:56)
[error] 	at sbt.internal.LibraryManagement$.resolve$1(LibraryManagement.scala:46)
[error] 	at sbt.internal.LibraryManagement$.$anonfun$cachedUpdate$12(LibraryManagement.scala:99)
[error] 	at sbt.util.Tracked$.$anonfun$lastOutput$1(Tracked.scala:68)
[error] 	at sbt.internal.LibraryManagement$.$anonfun$cachedUpdate$19(LibraryManagement.scala:112)
[error] 	at scala.util.control.Exception$Catch.apply(Exception.scala:224)
[error] 	at sbt.internal.LibraryManagement$.$anonfun$cachedUpdate$11(LibraryManagement.scala:112)
[error] 	at sbt.internal.LibraryManagement$.$anonfun$cachedUpdate$11$adapted(LibraryManagement.scala:95)
[error] 	at sbt.util.Tracked$.$anonfun$inputChanged$1(Tracked.scala:149)
[error] 	at sbt.internal.LibraryManagement$.cachedUpdate(LibraryManagement.scala:126)
[error] 	at sbt.Classpaths$.$anonfun$updateTask$5(Defaults.scala:2398)
[error] 	at scala.Function1.$anonfun$compose$1(Function1.scala:44)
[error] 	at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:39)
[error] 	at sbt.std.Transform$$anon$4.work(System.scala:66)
[error] 	at sbt.Execute.$anonfun$submit$2(Execute.scala:263)
[error] 	at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:16)
[error] 	at sbt.Execute.work(Execute.scala:272)
[error] 	at sbt.Execute.$anonfun$submit$1(Execute.scala:263)
[error] 	at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:174)
[error] 	at sbt.CompletionService$$anon$2.call(CompletionService.scala:37)
[error] 	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
[error] 	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
[error] 	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
[error] 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
[error] 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
[error] 	at java.lang.Thread.run(Thread.java:748)
[error] (update) sbt.librarymanagement.ResolveException: unresolved dependency: org.elasticsearch.client#x-pack-transport;5.6.2: not found

A similar issue elastic4s#775 applied this same fix, so this might be a useful addition here as well.