Fix docker staging build

temp fix to lock a dependency to a python 2.7 compatible version

Issue-ID: INT-1727
Signed-off-by: ChrisC <christophe.closset@intl.att.com>
Change-Id: I429c04e334a9afd106fb7430b9e3d3d91cced5cc
diff --git a/requirements.txt b/requirements.txt
index 238f461..b7f7914 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 robotframework==3.1.2
+pyrsistent==0.16.1
 selenium
 robotframework-seleniumlibrary
 robotframework-archivelibrary