[SPARK-9250] Make change-scala-version more helpful w.r.t. valid Scala versions

Author: François Garillot <francois@garillot.net>

Closes #7595 from huitseeker/issue/SPARK-9250 and squashes the following commits:

80a0218 [François Garillot] [SPARK-9250] Make change-scala-version's usage more explicit, introduce a -h|--help option.

dev/change-scala-version.sh

@@ -19,19 +19,23 @@
 
 set -e
 
+VALID_VERSIONS=( 2.10 2.11 )
+
 usage() {
-  echo "Usage: $(basename $0) <version>" 1>&2
+  echo "Usage: $(basename $0) [-h|--help] <version>
+where :
+  -h| --help Display this help text
+  valid version values : ${VALID_VERSIONS[*]}
+" 1>&2
   exit 1
 }
 
-if [ $# -ne 1 ]; then
+if [[ ($# -ne 1) || ( $1 == "--help") ||  $1 == "-h" ]]; then
   usage
 fi
 
 TO_VERSION=$1
 
-VALID_VERSIONS=( 2.10 2.11 )
-
 check_scala_version() {
   for i in ${VALID_VERSIONS[*]}; do [ $i = "$1" ] && return 0; done
   echo "Invalid Scala version: $1. Valid versions: ${VALID_VERSIONS[*]}" 1>&2