Update pip before installing dependencies

python.exe : You are using pip version 19.0.3, however version 19.1.1 is available.
At C:\MyProject\nuxeo-drive-client\tools\windows\deploy_jenkins_slave.ps1:301 char:2
+     & $Env:STORAGE_DIR\Scripts\python.exe $global:PYTHON_OPT $global: ...
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (You are using p...1 is available.:String) [], RemoteException
    + FullyQualifiedErrorId : NativeCommandError
 
You should consider upgrading via the 'python -m pip install --upgrade pip' command.