travis: Compile OSX releases with Xcode 7

Unfortunately what we're using right now, Xcode 8.2, cannot compile LLVM for OSX
10.7. We've done this historically and Gecko would like to maintain this
compabitiliby. This commit moves our release builders for OSX to using Xcode 7
which can compile LLVM for 10.7.

The builders running tests continue to use Xcode 8.2, however, because the LLDB
version with Xcode 7, 350, is blacklisted in running our LLDB tests. To continue
running LLDB tests we'll stick with Xcode 8.2.

.travis.yml

@@ -40,7 +40,10 @@ matrix:
     - env: IMAGE=x86_64-gnu-distcheck
     - env: IMAGE=x86_64-gnu-incremental
 
-    # OSX builders
+    # OSX builders running tests, these run the full test suite.
+    #
+    # Note that the compiler is compiled to target 10.8 here because the Xcode
+    # version that we're using, 8.2, cannot compile LLVM for OSX 10.7.
     - env: >
         RUST_CHECK_TARGET=check
         RUST_CONFIGURE_ARGS=--build=x86_64-apple-darwin
@@ -68,6 +71,12 @@ matrix:
       osx_image: xcode8.2
       install: *osx_install_sccache
 
+    # OSX builders producing releases. These do not run the full test suite and
+    # just produce a bunch of artifacts.
+    #
+    # Note that these are running in the `xcode7` image instead of the
+    # `xcode8.2` image as above. That's because we want to build releases for
+    # OSX 10.7 and `xcode7` is the latest Xcode able to compile LLVM for 10.7.
     - env: >
         RUST_CHECK_TARGET=dist
         RUST_CONFIGURE_ARGS="--build=i686-apple-darwin --enable-extended"
@@ -75,10 +84,9 @@ matrix:
         DEPLOY=1
         RUSTC_RETRY_LINKER_ON_SEGFAULT=1
         SCCACHE_ERROR_LOG=/tmp/sccache.log
-        MACOSX_DEPLOYMENT_TARGET=10.8
-        MACOSX_STD_DEPLOYMENT_TARGET=10.7
+        MACOSX_DEPLOYMENT_TARGET=10.7
       os: osx
-      osx_image: xcode8.2
+      osx_image: xcode7
       install: *osx_install_sccache
     - env: >
         RUST_CHECK_TARGET=dist
@@ -87,10 +95,9 @@ matrix:
         DEPLOY=1
         RUSTC_RETRY_LINKER_ON_SEGFAULT=1
         SCCACHE_ERROR_LOG=/tmp/sccache.log
-        MACOSX_DEPLOYMENT_TARGET=10.8
-        MACOSX_STD_DEPLOYMENT_TARGET=10.7
+        MACOSX_DEPLOYMENT_TARGET=10.7
       os: osx
-      osx_image: xcode8.2
+      osx_image: xcode7
       install: *osx_install_sccache
 
     # "alternate" deployments, these are "nightlies" but don't have assertions
@@ -105,10 +112,9 @@ matrix:
         DEPLOY_ALT=1
         RUSTC_RETRY_LINKER_ON_SEGFAULT=1
         SCCACHE_ERROR_LOG=/tmp/sccache.log
-        MACOSX_DEPLOYMENT_TARGET=10.8
-        MACOSX_STD_DEPLOYMENT_TARGET=10.7
+        MACOSX_DEPLOYMENT_TARGET=10.7
       os: osx
-      osx_image: xcode8.2
+      osx_image: xcode7
       install: *osx_install_sccache
 
 env: