update cocoapods to version 1.9.1 (latest stable) and use ubuntu:18_04

update cocoapods to version 1.8.4 (latest stable)

update cocoapods to version 1.7.2 (latest stable)

update cocoapods to version 1.7.1 (latest stable)

update cocoapods to version 1.6.1 (latest stable)

use specific version of cocoapods (1.5.3 is latest stable)