Bump scopt from 3.6.0 -> 3.7.0 (#877)

This bumps scopt from 3.6.0 to 3.7.0 to align with FIRRTL. FIRRTL
requires 3.7.0+ for added scopt methods that allow introspection
of options (e.g., examing the short options of a long option).
This bump avoids a compile-time warning due to the version
mistmatch.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

build.sbt

@@ -102,7 +102,7 @@ lazy val chiselSettings = Seq (
   libraryDependencies ++= Seq(
     "org.scalatest" %% "scalatest" % "3.0.1" % "test",
     "org.scalacheck" %% "scalacheck" % "1.13.4" % "test",
-    "com.github.scopt" %% "scopt" % "3.6.0"
+    "com.github.scopt" %% "scopt" % "3.7.0"
   ),
 
   // Tests from other projects may still run concurrently.