Adding a z-index to pipeline-history build cause popup.

The popup was going under the header when we have only 1 instance of pipeline run with more than 2 materials in the build-cause popup. I'm assuming that the bottom limit was the footer and hence the popup was moved up. Adding a z-index:5000 makes this popup appear over the header.
Merge pull request #607 from sachinsudheendra/bug-598

Adding a z-index to pipeline-history build cause popup.