Change Concourse VM to m4.xlarge
Change Concourse VM to m4.xlarge

Current Concourse instance type can't cope with parallel execution of all
test suites. This commit changes the type to m4.xlarge that doubles the
number of CPUs and available memory.

More info and reasoning here:
https://www.pivotaltracker.com/story/show/134551329

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf