Updating to use AbstractProject.getLastBuild() instead of AbstractProject.getSomeBuildWithWorkspace()

When running JMeter in EC2 Jenkins Slaves the trend graph will disappear because there is no builds with live workspaces.
Improving search for build directory by checking their is a PerformanceReport directory present before using it

This means that the trend graph will continue to show when a build is in progress
Merge branch 'performance-1.8-OmnifonePatch'

Conflicts:
	pom.xml
	src/main/java/hudson/plugins/performance/PerformanceProjectAction.java