fix exhaustive warnings. change "sealed abstract class" and "case object"

[xuwei-k]

before
https://travis-ci.org/scalikejdbc/scalikejdbc/jobs/210757869#L869

[warn] /home/travis/build/scalikejdbc/scalikejdbc/scalikejdbc-mapper-generator-core/src/main/scala/scalikejdbc/mapper/CodeGenerator.scala:327: match may not be exhaustive.
[warn] It would fail on the following input: GeneratorTemplate(_)
[warn]       val placeHolderPart: String = config.template match {
[warn]                                            ^
[warn] /home/travis/build/scalikejdbc/scalikejdbc/scalikejdbc-mapper-generator-core/src/main/scala/scalikejdbc/mapper/CodeGenerator.scala:347: match may not be exhaustive.
[warn] It would fail on the following input: GeneratorTemplate(_)
[warn]         (config.template match {
[warn]                 ^
[warn] /home/travis/build/scalikejdbc/scalikejdbc/scalikejdbc-mapper-generator-core/src/main/scala/scalikejdbc/mapper/CodeGenerator.scala:354: match may not be exhaustive.
[warn] It would fail on the following input: GeneratorTemplate(_)
[warn]         (config.template match {
[warn]                 ^
[warn] /home/travis/build/scalikejdbc/scalikejdbc/scalikejdbc-mapper-generator-core/src/main/scala/scalikejdbc/mapper/CodeGenerator.scala:361: match may not be exhaustive.
[warn] It would fail on the following input: GeneratorTemplate(_)
[warn]         (config.template match {
[warn]                 ^
[warn] /home/travis/build/scalikejdbc/scalikejdbc/scalikejdbc-mapper-generator-core/src/main/scala/scalikejdbc/mapper/CodeGenerator.scala:413: match may not be exhaustive.
[warn] It would fail on the following input: GeneratorTemplate(_)
[warn]       val placeHolderPart: String = config.template match {
[warn]                                            ^
[warn] /home/travis/build/scalikejdbc/scalikejdbc/scalikejdbc-mapper-generator-core/src/main/scala/scalikejdbc/mapper/CodeGenerator.scala:425: match may not be exhaustive.
[warn] It would fail on the following input: GeneratorTemplate(_)
[warn]       val wherePart = config.template match {
[warn]                              ^