Remove 2.11 scalaVersion to fix 2.12 build

[iantabolt]

Running Epic 0.4.4 on Scala 2.12, I'm running into errors like this:

... snip ...
Caused by: java.lang.BootstrapMethodError: java.lang.NoSuchMethodError: scala.Array$.$anonfun$fill$1$adapted(Lscala/Array$;ILscala/Function0;Lscala/reflect/ClassTag;Ljava/lang/Object;)Ljava/lang/Object;
    at epic.sequences.CRF$.viterbi(CRF.scala:309)
    at epic.sequences.CRF.bestSequence(CRF.scala:42)
    at epic.sequences.CRF.bestSequence$(CRF.scala:41)
    at epic.sequences.CRFInference.bestSequence(CRFModel.scala:58)
    at ... snip ...
    ... 45 more
Caused by: java.lang.NoSuchMethodError: scala.Array$.$anonfun$fill$1$adapted(Lscala/Array$;ILscala/Function0;Lscala/reflect/ClassTag;Ljava/lang/Object;)Ljava/lang/Object;
    at java.lang.invoke.MethodHandleNatives.resolve(Native Method)
    at java.lang.invoke.MemberName$Factory.resolve(MemberName.java:975)
    at java.lang.invoke.MemberName$Factory.resolveOrFail(MemberName.java:1000)
    at java.lang.invoke.MethodHandles$Lookup.resolveOrFail(MethodHandles.java:1394)
    at java.lang.invoke.MethodHandles$Lookup.linkMethodHandleConstant(MethodHandles.java:1750)
    at java.lang.invoke.MethodHandleNatives.linkMethodHandleConstant(MethodHandleNatives.java:477)
    ... 68 more

The error is consistent with having Scala 2.11 on the classpath of a 2.12 bulid. Epic has scalaVersion := 2.11 along with crossScalaVersions which upon removing fixes my classpath issue.

I also bumped 2.12 to the latest (2.12.10) and simplified the scala-xml expression without the scalaVersion match.

[dlwh]

thanks!