ci: Update Travis OSX builders

Looks like Travis [has announced][blog] that our current `xcode8.2` image is
being deprecated and the recommended Xcode 7 image is `xcode7.3`. This updates
us to these ahead of time to make sure we can shake out any bugs, if any.

[blog]: https://blog.travis-ci.com/2017-10-16-a-new-default-os-x-image-is-coming

Author: alexcrichton

.travis.yml

@@ -36,7 +36,7 @@ matrix:
         NO_LLVM_ASSERTIONS=1
         NO_DEBUG_ASSERTIONS=1
       os: osx
-      osx_image: xcode7
+      osx_image: xcode7.3
       if: branch = auto
 
     # macOS builders. These are placed near the beginning because they are very
@@ -57,7 +57,7 @@ matrix:
         NO_LLVM_ASSERTIONS=1
         NO_DEBUG_ASSERTIONS=1
       os: osx
-      osx_image: xcode8.2
+      osx_image: xcode8.3
       if: branch = auto
 
     - env: >
@@ -71,7 +71,7 @@ matrix:
         NO_LLVM_ASSERTIONS=1
         NO_DEBUG_ASSERTIONS=1
       os: osx
-      osx_image: xcode8.2
+      osx_image: xcode8.3
       if: branch = auto
 
     # OSX builders producing releases. These do not run the full test suite and
@@ -91,7 +91,7 @@ matrix:
         NO_LLVM_ASSERTIONS=1
         NO_DEBUG_ASSERTIONS=1
       os: osx
-      osx_image: xcode7
+      osx_image: xcode7.3
       if: branch = auto
 
     - env: >
@@ -105,7 +105,7 @@ matrix:
         NO_LLVM_ASSERTIONS=1
         NO_DEBUG_ASSERTIONS=1
       os: osx
-      osx_image: xcode7
+      osx_image: xcode7.3
       if: branch = auto
 
     # Linux builders, remaining docker images