Update requirements.txt

Author: Vincent Royer

docs/elassandra/requirements.txt

@@ -1,14 +1,14 @@
 alabaster==0.7.10
-Babel==2.5.1
+babel>=2.9.1
 certifi==2017.7.27.1
 chardet==3.0.4
 CommonMark==0.5.4
 docutils==0.14
 idna==2.6
 imagesize==0.7.1
-Jinja2==2.10.1
+jinja2>=2.11.3
 MarkupSafe==1.0
-Pygments==2.2.0
+Pygments>=2.7.4
 pytz==2017.2
 recommonmark==0.4.0
 requests==2.21.0
@@ -18,4 +18,4 @@ Sphinx==1.6.4
 sphinx-rtd-theme==0.2.4
 sphinxcontrib-websupport==1.0.1
 typing==3.6.2
-urllib3==1.24.3
+urllib3>=1.26.5