Update pod spec version number.

ResearchKit.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = 'ResearchKit'
-  s.version      = '1.3.0'
+  s.version      = '1.3.1'
   s.summary      = 'ResearchKit is an open source software framework that makes it easy to create apps for medical research or for other research projects.'
   s.homepage     = 'https://www.github.com/ResearchKit/ResearchKit'
   s.documentation_url = 'http://researchkit.github.io/docs/'

ResearchKit/Common/ORKHelpers.h

@@ -269,7 +269,7 @@ id ORKDynamicCast_(id x, Class objClass);
 
 #define ORKDynamicCast(x, c) ((c *) ORKDynamicCast_(x, [c class]))
 
-const CGFloat ORKScrollToTopAnimationDuration;
+extern const CGFloat ORKScrollToTopAnimationDuration;
 
 ORK_INLINE CGFloat
 ORKCGFloatNearlyEqualToFloat(CGFloat f1, CGFloat f2) {
@@ -285,7 +285,7 @@ void ORKValidateArrayForObjectsOfClass(NSArray *array, Class expectedObjectClass
 
 void ORKRemoveConstraintsForRemovedViews(NSMutableArray *constraints, NSArray *removedViews);
 
-const CGFloat ORKCGFloatInvalidValue;
+extern const CGFloat ORKCGFloatInvalidValue;
 
 void ORKAdjustPageViewControllerNavigationDirectionForRTL(UIPageViewControllerNavigationDirection *direction);