[ios] Updated podspec for 3.1.0-pre.1 and iOS 8+

The pod now requires iOS 8 and above, since iOS 7 lacks support for dynamic frameworks. iOS 7 support can be obtained by downloading and installing the framework manually. Bumped the version number.

ios/Mapbox-iOS-SDK.podspec

@@ -1,24 +1,24 @@
 Pod::Spec.new do |m|
 
   m.name    = 'Mapbox-iOS-SDK'
-  m.version = '3.0.1-symbols'
+  m.version = '3.1.0-pre.1-symbols'
 
   m.summary           = 'Open source vector map solution for iOS with full styling capabilities.'
   m.description       = 'Open source, OpenGL-based vector map solution for iOS with full styling capabilities and Cocoa Touch APIs.'
   m.homepage          = 'https://www.mapbox.com/ios-sdk/'
-  m.license           = { :type => 'BSD', :file => 'Mapbox.framework/LICENSE.md' }
+  m.license           = { :type => 'BSD', :file => 'LICENSE.md' }
   m.author            = { 'Mapbox' => 'mobile@mapbox.com' }
   m.screenshot        = 'https://raw.githubusercontent.com/mapbox/mapbox-gl-native/master/ios/screenshot.png'
   m.social_media_url  = 'https://twitter.com/mapbox'
   m.documentation_url = 'https://www.mapbox.com/ios-sdk/'
 
   m.source = {
-    :http => "https://mapbox.s3.amazonaws.com/mapbox-gl-native/ios/builds/Mapbox.framework-#{m.version.to_s}.zip",
+    :http => "https://mapbox.s3.amazonaws.com/mapbox-gl-native/ios/builds/mapbox-ios-sdk-#{m.version.to_s}-dynamic.zip",
     :flatten => true
   }
 
   m.platform              = :ios
-  m.ios.deployment_target = '7.0'
+  m.ios.deployment_target = '8.0'
 
   m.requires_arc = true