Update spark_ec2.py

"Latest Spark AMI: ami-4c5fcd25
Launching instances...
ERROR:boto:400 Bad Request
ERROR:boto:<?xml version="1.0" encoding="UTF-8"?>
<Response><Errors><Error><Code>Unsupported</Code><Message>The requested Availability Zone is currently constrained and we are no longer accepting new customer requests for t1/m1/c1/m2/m3 instance types. Please retry your request by not specifying an Availability Zone or choosing us-east-1a, us-east-1d, us-east-1e.</Message></Error></Errors><RequestID>0b081ec9-2ea7-4470-baf3-7e3d87ba5d16</RequestID></Response>"