Fix bug parsing dependencies from feature.xml

[seflue]

Return a list of string representations of XML attributes instead of the
attributes itself (which caused a type mismatch) from parsing method.

The bug caused following error when importing features via Buildship in Eclipse:

A problem occurred configuring project ':my_feature'.
No signature of method: de.monkeyworks.buildmonkey.dependency.ManifestDependencyPlugin$_addTasksForResolveDependencies_closure1$_closure8$_closure12.doCall() is applicable for argument types: (groovy.util.slurpersupport.Attributes) values: [my.plugin.url]
Possible solutions: doCall(java.lang.String), findAll(), findAll(), isCase(java.lang.Object), isCase(java.lang.Object)
...
Caused by: groovy.lang.MissingMethodException: No signature of method: de.monkeyworks.buildmonkey.dependency.ManifestDependencyPlugin$_addTasksForResolveDependencies_closure1$_closure8$_closure12.doCall() is applicable for argument types: (groovy.util.slurpersupport.Attributes) values: [my.plugin.url]
Possible solutions: doCall(java.lang.String), findAll(), findAll(), isCase(java.lang.Object), isCase(java.lang.Object)
at de.monkeyworks.buildmonkey.dependency.ManifestDependencyPlugin$_addTasksForResolveDependencies_closure1$_closure8.doCall(ManifestDependencyPlugin.groovy:106)