requirements.txt: Update libraries

Switch to a personal fork of SolrClient so that we can use kazoo 2.5.0
and get rid of the error about the 'async' keyword on Python 3.7. Also
this bumps some of the other libraries to their latest versions.

requirements.txt

@@ -1,4 +1,4 @@
-certifi==2018.8.24
+certifi==2018.10.15
 chardet==3.0.4
 falcon==1.4.1
 gunicorn==19.9.0
@@ -6,7 +6,7 @@ idna==2.7
 kazoo==2.5.0
 psycopg2-binary==2.7.5
 python-mimeparse==1.6.0
-requests==2.19.1
+requests==2.20.0
 six==1.11.0
-SolrClient==0.2.1
-urllib3==1.23
+-e git://github.com/alanorth/SolrClient.git@c629e3475be37c82770b2be61748be7e29882648#egg=SolrClient
+urllib3==1.24