Update conflicting python dependencies (elastic#8961) (elastic#9170)

Through updating requests to 2.20.0 some of the python dependencies have become incompatible.

This was introduced in elastic#8808. Not sure if it actually caused issues but warnings were shown when running make update.

(cherry picked from commit 49cf866)

libbeat/tests/system/requirements.txt

@@ -3,9 +3,9 @@ backports.ssl-match-hostname==3.5.0.1
 cached-property==1.4.2
 certifi==2018.1.18
 chardet==3.0.4
-docker==3.2.1
-docker-compose==1.21.0
-docker-pycreds==0.2.2
+docker==3.5.1
+docker-compose==1.23.1
+docker-pycreds==0.3.0
 dockerpty==0.4.1
 docopt==0.6.2
 elasticsearch==6.2.0