Attempting to fix Travis build
Attempting to fix Travis build

- Switching from `pod spec lint` to `pod lib lint`
- This should prevent lint failures from not yet having new version tags on Github

ibm-bluemix-mobile-services/bms-clientsdk-swift-core,ibm-bluemix-mobile-services/bms-clientsdk-swift-core,ibm-bluemix-mobile-services/bms-clientsdk-swift-core