Bump version number to 1.15.0 (grpc#1584)

Motivation:

We plan on tagging a release soon.

Modifications:

- Bump the version to 1.15.0

Result:

The version in the default user-agent string will match the released
version.

Author: glbrntt

Sources/GRPC/Version.swift

@@ -19,10 +19,10 @@ internal enum Version {
   internal static let major = 1
 
   /// The minor version.
-  internal static let minor = 14
+  internal static let minor = 15
 
   /// The patch version.
-  internal static let patch = 2
+  internal static let patch = 0
 
   /// The version string.
   internal static let versionString = "\(major).\(minor).\(patch)"