upgrade celery to 4.2.0

celery 4.2.0 no longer requires boto2, so we can get rid of that
patched version.

requirements.txt

@@ -4,7 +4,7 @@
 # with package version changes made in requirements-app.txt
 
 cffi==1.11.5
-celery==3.1.26.post2 # pyup: <4
+celery==4.2.1
 docopt==0.6.2
 Flask-Bcrypt==0.7.1
 flask-marshmallow==0.9.0
@@ -31,14 +31,11 @@ botocore<1.11.0
 
 git+https://github.com/alphagov/notifications-utils.git@30.4.0#egg=notifications-utils==30.4.0
 
-git+https://github.com/alphagov/boto.git@2.43.0-patch3#egg=boto==2.43.0-patch3
-
 ## The following requirements were added by pip freeze:
 alembic==1.0.0
-amqp==1.4.9
-anyjson==0.3.3
+amqp==2.3.2
 bcrypt==3.1.4
-billiard==3.3.0.23
+billiard==3.5.0.4
 bleach==2.1.3
 boto3==1.6.16
 certifi==2018.8.24
@@ -54,7 +51,7 @@ idna==2.7
 itsdangerous==0.24
 Jinja2==2.10
 jmespath==0.9.3
-kombu==3.0.37
+kombu==4.2.1
 Mako==1.0.7
 MarkupSafe==1.0
 mistune==0.8.3
@@ -77,6 +74,6 @@ six==1.11.0
 smartypants==2.0.1
 statsd==3.2.2
 urllib3==1.23
+vine==1.1.4
 webencodings==0.5.1
 Werkzeug==0.14.1
-wheel==0.24.0