Fix: Using SCNQuaternion constructor instead of tuple-assignment

* In order to quell the warning in Xcode 9 that _“initializer for struct 'SCNVector4' must use "self.init(...)" or "self = ..." because the struct was imported from C”_, switched from doing `x`/`y`/`z`/`w`-tuple assignment to just using the 4-arg constructor.  
	▸ It appears the 4-arg constructor has always been there; I just didn't use it for some reason before.

Also:

* Bumped project version to 1.3.1.

SCNMathExtensions.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name = 'SCNMathExtensions'
-  s.version = '1.3.0'
+  s.version = '1.3.1'
   s.summary = 'A Swift µ-Library of Math Extensions for SCNVector3, SCNQuaternion, SCNMatrix4'
   s.description = "A Swift micro-library that provides commonplace math extensions for SceneKit's SCNVector3, SCNQuaternion, & SCNMatrix4 types, and bridging to/from GLKVector3, simd.float3, GLKQuaternion, GLKMatrix4, & simd.float4x4."
   s.homepage = 'https://github.com/capnslipp/SCNMathExtensions'

SCNMathExtensions.xcodeproj/project.pbxproj

@@ -260,7 +260,7 @@
 			buildSettings = {
 				COMBINE_HIDPI_IMAGES = YES;
 				COPY_PHASE_STRIP = NO;
-				CURRENT_PROJECT_VERSION = 1.3.0;
+				CURRENT_PROJECT_VERSION = 1.3.1;
 				DEBUG_INFORMATION_FORMAT = dwarf;
 				DYLIB_INSTALL_NAME_BASE = "@rpath";
 				ENABLE_NS_ASSERTIONS = YES;
@@ -282,7 +282,7 @@
 			buildSettings = {
 				COMBINE_HIDPI_IMAGES = YES;
 				COPY_PHASE_STRIP = YES;
-				CURRENT_PROJECT_VERSION = 1.3.0;
+				CURRENT_PROJECT_VERSION = 1.3.1;
 				DEBUG_INFORMATION_FORMAT = "dwarf-with-dsym";
 				DYLIB_INSTALL_NAME_BASE = "@rpath";
 				GCC_OPTIMIZATION_LEVEL = s;

Sources/SCNMathExtensions.swift

@@ -339,7 +339,7 @@ extension SCNQuaternion
 			// looks to be explained at: http://lolengine.net/blog/2013/09/18/beautiful-maths-quaternion-from-vectors
 			let s = sqrt((1.0 + dotProduct) * 2.0)
 			let xyz = aNormal.crossProduct(bNormal) / s
-			(self.x, self.y, self.z, self.w) = (xyz.x, xyz.y, xyz.z, (s * 0.5))
+			self = SCNQuaternion(xyz.x, xyz.y, xyz.z, (s * 0.5))
 		}
 	}