Increased git depth to 10000

Cloning the Iris repo on Travis CI has recently started failing with the following error:

`The command "git clone --depth=0 git://github.com/SciTools/iris.git SciTools/iris" failed.`

The simple fix is to increase the git depth to significantly greater (in this case 10000 commits) than the total number of Iris commits. This will give time for a fix to requiring all the git history to be determined.