travis: Tweak OSX image configuration

Somewhere between https://travis-ci.org/rust-lang/rust/jobs/192352185 and
https://travis-ci.org/rust-lang/rust/jobs/192440181 it looks like our
configuration for a newer OSX image was lost as LLDB has reverted itself back to
350. This fix appeared to work for the libc crate so let's see if we can
configure it to work for the rust repo as well.

.travis.yml

@@ -8,8 +8,6 @@ git:
   depth: 1
   submodules: false
 
-osx_image: xcode8.2
-
 matrix:
   include:
     # Linux builders, all docker images
@@ -41,6 +39,7 @@ matrix:
         RUST_CONFIGURE_ARGS=--build=x86_64-apple-darwin
         SRC=.
       os: osx
+      osx_image: xcode8.2
       before_script: &osx_before_script >
         ulimit -c unlimited
       install: &osx_install_sccache >
@@ -59,6 +58,7 @@ matrix:
         SRC=.
         DEPLOY=1
       os: osx
+      osx_image: xcode8.2
       before_script: *osx_before_script
       install: *osx_install_sccache
       after_failure: *osx_after_failure
@@ -67,6 +67,7 @@ matrix:
         RUST_CONFIGURE_ARGS=--build=x86_64-apple-darwin --disable-rustbuild
         SRC=.
       os: osx
+      osx_image: xcode8.2
       before_script: *osx_before_script
       install: *osx_install_sccache
       after_failure: *osx_after_failure
@@ -76,6 +77,7 @@ matrix:
         SRC=.
         DEPLOY=1
       os: osx
+      osx_image: xcode8.2
       before_script: *osx_before_script
       install: *osx_install_sccache
       after_failure: *osx_after_failure