Bump version to 1.3.1 for Swift <4.2 build fix

Author: thomasvl

Sources/SwiftProtobuf/Version.swift

@@ -21,7 +21,7 @@ public struct Version {
   /// Minor version.
   public static let minor = 3
   /// Revision number.
-  public static let revision = 0
+  public static let revision = 1
 
   /// String form of the version number.
   public static let versionString = "\(major).\(minor).\(revision)"

SwiftProtobuf.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name = 'SwiftProtobuf'
-  s.version = '1.3.0'
+  s.version = '1.3.1'
   s.license = { :type => 'Apache 2.0', :file => 'LICENSE.txt' }
   s.summary = 'Swift Protobuf code generator plugin and runtime library'
   s.homepage = 'https://github.com/apple/swift-protobuf'

SwiftProtobuf.xcodeproj/project.pbxproj

@@ -2266,7 +2266,7 @@
 				CLANG_WARN_UNREACHABLE_CODE = YES;
 				CLANG_WARN__DUPLICATE_METHOD_MATCH = YES;
 				COMBINE_HIDPI_IMAGES = YES;
-				CURRENT_PROJECT_VERSION = 1.3.0;
+				CURRENT_PROJECT_VERSION = 1.3.1;
 				DEBUG_INFORMATION_FORMAT = "dwarf-with-dsym";
 				DYLIB_CURRENT_VERSION = 1;
 				DYLIB_INSTALL_NAME_BASE = "@rpath";
@@ -2323,7 +2323,7 @@
 				CLANG_WARN__DUPLICATE_METHOD_MATCH = YES;
 				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_CURRENT_VERSION = 1;
 				DYLIB_INSTALL_NAME_BASE = "@rpath";