spark version is taken from the environment variable SPARK_VERSION

Author: Alok  Singh <singhal@Aloks-MacBook-Pro.local>
Author: Alok  Singh <singhal@aloks-mbp.usca.ibm.com>

Closes #6944 from aloknsingh/aloknsingh_spark_jiras and squashes the following commits:

ed607bd [Alok  Singh] [SPARK-8111][SparkR] As per suggestion, 1) using the version from sparkContext rather than the Sys.env. 2) change "Welcome to SparkR!" to "Welcome to" followed by Spark logo and version
acd5b85 [Alok  Singh] fix the jira SPARK-8111 to add the spark version and logo. Currently spark version is taken from the environment variable SPARK_VERSION