fix requirements (GoogleCloudPlatform#2014)

Author: alixhami

bigquery/datalab-migration/requirements.txt

@@ -1,5 +1,5 @@
 google-cloud-bigquery[pandas,pyarrow]==1.8.1
 datalab==1.1.4
-ipython==7.2.0; python_version > '2.7'
-ipython==5.5; python_version == '2.7'
-google-cloud-monitoring==0.28.1
+ipython==7.2.0; python_version > "2.7"
+ipython<=5.5; python_version == "2.7"
+google-cloud-monitoring<=0.28.1