Updated Appveyor build script with the Rust 1.15.1 dependency

Author: Razican

.appveyor.yml

@@ -13,11 +13,11 @@ environment:
     MSYS_BITS: 64
   - TARGET: beta-i686-pc-windows-gnu
     MSYS_BITS: 32
-  - TARGET: 1.14.0-x86_64-pc-windows-msvc
-  - TARGET: 1.14.0-i686-pc-windows-msvc
-  - TARGET: 1.14.0-x86_64-pc-windows-gnu
+  - TARGET: 1.15.1-x86_64-pc-windows-msvc
+  - TARGET: 1.15.1-i686-pc-windows-msvc
+  - TARGET: 1.15.1-x86_64-pc-windows-gnu
     MSYS_BITS: 64
-  - TARGET: 1.14.0-i686-pc-windows-gnu
+  - TARGET: 1.15.1-i686-pc-windows-gnu
     MSYS_BITS: 32
 
 install: