Install elm locally if missing

[francisdb]

Just pushed my code and this came out of our CI

java.io.IOException: Cannot run program "elm-make": error=2, No such file or directory
    at java.lang.ProcessBuilder.start(ProcessBuilder.java:1048)
    at sbt.SimpleProcessBuilder.run(ProcessImpl.scala:349)
    at sbt.AbstractProcessBuilder.run(ProcessImpl.scala:128)
    at sbt.AbstractProcessBuilder$$anonfun$runBuffered$1.apply(ProcessImpl.scala:159)
    at sbt.AbstractProcessBuilder$$anonfun$runBuffered$1.apply(ProcessImpl.scala:159)
    at sbt.elm.SbtElm$$anon$1.buffer(SbtElm.scala:190)
    at sbt.AbstractProcessBuilder.runBuffered(ProcessImpl.scala:159)
    at sbt.AbstractProcessBuilder.$bang(ProcessImpl.scala:156)
    at sbt.elm.SbtElm$.doCompile(SbtElm.scala:178)
    at sbt.elm.SbtElm$$anonfun$6$$anonfun$8.apply(SbtElm.scala:127)
    at sbt.elm.SbtElm$$anonfun$6$$anonfun$8.apply(SbtElm.scala:119)
...

Would be nice if the plugin could optionally install elm like the other web plugins can have their dependencies. This would probably require an elm webjar?
An other solution for us would be calling npm install elm and using the binaries from the node_modules folder. But then of course you need npm installed (moving the problem to the next dependency)

By the way you should probably set up travis and see how you can fix it there?