When you dont define an iOS version, 4.3 is assumed as the default by cocoapods
For example if you define the Podfile as follow platform :ios, '5.0'
the minimum iOS version for your porject will be 5.0
In your case it means that Nimbus has a higher iOS version number defined to be the minimum in their podspec. Depending on your project minimum target either update your Podfile to support a higher version or check if previous versions of Nimbus are compatible with ios 4.3
for defining a specific version for a pod use pod 'Nimbus', '0.X.Y'
where X and Y are the version numbers