Could not find implicit value for parameter encoder compiler error on Scala 2.12

[hectcastro]

The compilation phase for Scala 2.12 appears to be failing:

[error] /home/circleci/project/shared/src/test/scala/ast/codec/tree/MamlExpressionTreeCodecSpec.scala:19: could not find implicit value for parameter encoder: io.circe.Encoder[com.azavea.maml.ast.Expression]
[error]       val encoded = ast.asJson.noSpaces
[error]                         ^
[error] /home/circleci/project/shared/src/test/scala/ast/codec/tree/MamlExpressionTreeCodecSpec.scala:28: overloaded method value fail with alternatives:
[error]   (cause: Throwable)(implicit pos: org.scalactic.source.Position)Nothing <and>
[error]   (message: String)(implicit pos: org.scalactic.source.Position)Nothing
[error]  cannot be applied to (Any)
[error]           fail(f)
[error]           ^
[error] two errors found
[warn] two warnings found
[info] Compiling 8 Scala sources to /home/circleci/project/jvm/target/scala-2.12/test-classes...
[info] Compiling 4 Scala sources to /home/circleci/project/spark/target/scala-2.12/classes...
[error] /home/circleci/project/shared/src/test/scala/ast/codec/tree/MamlExpressionTreeCodecSpec.scala:19: could not find implicit value for parameter encoder: io.circe.Encoder[com.azavea.maml.ast.Expression]
[error]       val encoded = ast.asJson.noSpaces
[error]                         ^
[error] /home/circleci/project/shared/src/test/scala/ast/codec/tree/MamlExpressionTreeCodecSpec.scala:28: overloaded method value fail with alternatives:
[error]   (cause: Throwable)(implicit pos: org.scalactic.source.Position)Nothing <and>
[error]   (message: String)(implicit pos: org.scalactic.source.Position)Nothing
[error]  cannot be applied to (Any)
[error]           fail(f)
[error]           ^
[error] two errors found

• [ ] Before resolving this issue, ensure that 2.12 support is re-enabled on CI.

See: https://circleci.com/gh/geotrellis/maml/191

[rbreslow]

I believe this was resolved by #95.